How would a Catholic explain the absence of the mass in the Epistle to the Hebrews?

The Sacrifice of the Mass is not a repetition over and over of Christ's sacrifice.

The proper understanding is that it is the one sacrifice of Christ made present.

This is exactly what we covered two weeks ago in my Religious Ed. class so bear with my short Catechism

1. Q: Why is Jesus a priest like Melchizedek?

   A: Because he offered bread and wine to God in (Jeru)salem

2. Q: Are Catholic priests still in the order of Melchizedek?

   A: Yes

3. Q: Who offers the sacrifice in the Mass?

   A: Jesus

4. Q: What is the sacrifice being offered?

   A: Jesus' Sacrifice

5. Q: Is this the same sacrifice as the Cross and the Last Supper?

   A: Yes (this is being "offered" each time we celebrate Mass)

6. Q: Does this mean we are re-sacrificing Jesus each time we celebrate Mass?

   A: No, it the One Sacrifice of Jesus is made present on the alter at Mass.

and, this is because, as Hebrews says:

Hebrews 10:1;11-14 (DRA)
¹ For the law having a shadow of the good things to come, not the very image of the things; by the selfsame sacrifices which they offer continually every year, can never make the comers thereunto perfect:

¹¹  And every priest indeed standeth daily ministering, and often offering the same sacrifices, which can never take away sins. ¹²  But this man offering one sacrifice for sins, for ever sitteth on the right hand of God, ¹³  From henceforth expecting, until his enemies be made his footstool. ¹⁴  For by one oblation he hath perfected for ever them that are sanctified.

And this, is the One Sacrifice that is offered by priests all over the world hundreds of thousands of times a day. A good follow up question (and one that I don't quite understand) "is what do Catholics mean when they say 'offered'"?

The Epistle to the Hebrews is not aware of the Mass because it is not a person. It's a letter on a certain subject to certain people. It talks about the High Priesthood; it isn't an instruction manual on what today has come to be called the Mass. Not all teachings will be in all books.

Hebrews isn't an instruction manual for saying the Mass. It's a letter directed to Jewish Christians in danger of apostasy (Hebrews 2:1), explaining the sacrifice of Christ with particular emphasis on themes in the Old Testament that would have been very familiar to the audience. It is by not intended to be comprehensive ("I have written to you rather briefly", Hebrews 13:22), but it does mean to explore in-depth a particular theme. The author invokes Melchizedek and the High Priesthood, and stresses the importance of Christ's precious blood, in comparison to the blood of animals. He calls the bloody sacrifices that came before, now abolished, mere shadows of Christ's perpetual sacrifice. The High Priesthood of Christ is the primary message, and it's directed to those who have lost clear sight of even basic teachings:

Hebrews 5
[...] 8 Son though he was, he learned obedience from what he suffered; 9 and when he was made perfect, he became the source of eternal salvation for all who obey him, 10 declared by God high priest according to the order of Melchizedek. 11 About this we have much to say, and it is difficult to explain, for you have become sluggish in hearing. 12 Although you should be teachers by this time, you need to have someone teach you again the basic elements of the utterances of God. You need milk, [and] not solid food.

So he would prefer to discuss the basics, but the problem, he judges, has now advanced far beyond the basics, and can only be remedied by detailed discussion of the High Priesthood of Christ.

Not all teachings will be repeated in all books, and the books will have different themes and will emphasize different points. For example, the following teaching (on an aspect of what today is called the Mass) is also not present in Hebrews:

1 Corinthians 11
26 For as often as you eat this bread and drink the cup, you proclaim the death of the Lord until he comes. 27 Therefore whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord. 28 A person should examine himself, and so eat the bread and drink the cup. 29 For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.

Apparently, there is something very important about not eating the bread, not drinking the cup. Does "drinking" mean that you should not try to have faith if you are unworthy? That you should not do good if you are unworthy? Or that you should starve until you are worthy? No, it means you must literally not drink and eat something (Someone, Catholics might say, but let's not get into that), because "proclaiming the death of the Lord" with His blood on your own hands is going to end badly for you. Where is this vital teaching in Hebrews? Nowhere, because in Hebrews something else was at issue: Jewish Christians focused on bloody sacrifices, and not the sacrifice of the High Priest.

For the record, some Catholics do use Hebrews to talk extensively on the subject of the Eucharist. I found, for example, "The Eucharist as the Meal of Melchizedek". Two relevant paragraphs can be found in that document by searching for "clean priesthood" and "brick in the face".