How does the idea of Bodhisatva in Mahayana mesh with anattā?

One idea is that having a view-of-self is conducive to suffering (viewing a conditioned, impermanent and unsatisfactory thing as "self"), and therefore it's better to choose to develop a view of non-self.

One way to practice this view, is to view yourself as being like others (and others as being like yourself): because that avoids (or tries to avoid) putting undue emphasis on "me".

Perhaps a view like this (i.e "treat others as yourself, with compassion") is common to many human cultures: see Golden Rule.

If "you" wanted to attain liberation and you're eventually successful, don't you think it makes sense to help others do the same? If a bodhisattva is defined as one who has Bodhicitta, don't you find that's identical to the thought process of the Buddha soon after he attained his enlightenment?

Ayacana Sutta

Then the Blessed One, having understood Brahma's invitation, out of compassion for beings, surveyed the world with the eye of an Awakened One. As he did so, he saw beings with little dust in their eyes [etc.]

Maybe the view of "emptiness" is to help you avoid clinging to the world: but it would be going too far, it would be untrue to say, "there are no sentient beings in the world, who suffer and want enlightenment."

As to the first paragraph regarding selfishness of Theravada, I would say it's wrong view or misunderstanding from the Mahayana counter part as my explanations later would show.

As to the second paragraph regarding compassion to oneself & others and difference between Mahayana and Theravada see my post there.

The middle way is seeing both ultimate & conventional reality and acting according to the reality of the situation, depending on personal characteristics. Acting according to just either one of realities is an extreme.

When true compassion arises in one there is only concern for the relieving of suffering of the other, there isn't a sense of self that I'm doing this, but rather this needs to be done.

And for an accomplished person, an Anagami (whose sense of self is much reduced) and higher, there isn't a feeling of one doing it, nor is there a sense of achievement, but for lesser beings: feel-good to pride.

So Theravadin Anagamis and above are true Bodhisattvas, but there's one caveat: most of those who chose the Theravadin path have concerns for me first others second, characteristics, make-up or tendencies. Those whose tendencies are others first and me second, are attracted to the Mahayana paths.

EDIT:

One of the suttas says, there are four types of persons:

1) One who does no good to himself or others. ( probably won't practice)

2) One who does good for himself and not for others. ( will probably choose Theravada)

3) One who does good for others but not for himself. ( Will probably choose Mahayana )

4) One who does good for others and himself. ( Accomplished persons).

This is related to this question.

The anatta concept doesn't say that there is no self - it says "not self" as in "all phenomena is not self". There is a self, but it is not permanent and is not independent from the inter-workings of the senses, sensation, perception, mind and consciousness.

For e.g. a static image that is projected on the wall - it is a result of light projected onto a wall from a projector. If you dig deeper, you can explain how a projector works, how electronics and electricity works. The image does not exist permanently and independently from the wall or projector or electricity. But this does not mean that the image does not exist. Surely it exists.

Also, to quote this answer, anatta relates to absolute truth, however, the self that exists relates to relative truth.

While you have not yet attained Nirvana (Bodhisattvas included), suffering is still experienced, hence the self still persists and rebirth continues to take place. When you knock your head on the wall and say "ouch", you know that you are still subject to suffering.

In the Acela Sutta, Kassapa asks the Buddha whether the suffering that one experiences, is caused by oneself, is caused by another, is caused both by oneself and another, is not caused by oneself or another but arises spontaneously, or it doesn't exist? The Buddha says that suffering exists, but it is not useful for us to speculate "who" ultimately caused it, from the perspective of absolute truth. If the self caused the suffering, then it is the eternalistic argument. If no one caused it, then it is the nihilistic argument. But instead, the Buddha taught Kassapa the middle path, which is dependent origination.

So, it is also not as easy as there is no self at all, or that there is definitely a self, and trying to link the effort to liberate oneself or other beings to one of these. Rather, the Buddha prefers us to ignore that part ("Don't say that, Kassapa") and focus on dependent origination.

Compassion by itself can be a contributor towards liberation from suffering according to the Nissaraniya Sutta:

It's impossible, there is no way that — when compassion has been developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, and well-undertaken as an awareness-release — viciousness would still keep overpowering the mind. That possibility doesn't exist, for this is the escape from viciousness: compassion as an awareness-release.'

I don't think Theravada lacks compassion, as it teaches the practice of the Brahmaviharas. But is it selfish to liberate oneself before trying to liberate others? Well, I think in Theravada there are the four stages of enlightenment (which can last up to 7 births from the first stage), plus an Arahant can continue to teach, till parinibbana, so there is still plenty of chance for one to contribute towards the liberation of others, but not quite till the hells are empty.