score:7
When you try to purchase from the MVV ticket machine, you should look for options for buying an Isarcard and not try to enter your destination. If you are trying to purchase from a DB ticket machine, you should select the MVV tab, but I'm not sure if there is an option to buy the Isar card from here. But all S,U stations have the MVV ticketing machines and you should be able to buy the Isarcard from there.
With the Isarcard option, you get an option to buy a weekly ticket for the zone you would want the ticket to be valid. More info here. I found this video which shows the step-by-step procedure to buy a weekly ticket. :)
It is unfortunately not possible to buy the Isarcard online. But you can purchase them either at the ticket machines of MVV or the MVV ticket centers in Hauptbahnhof/ Ostbahnhof etc.