VVT has all of the connections available across the whole of Tirol.
You can also download their app (one for connections and another for tickets)
If you are coming from Innsbruck Airport then you need to take the bus from the airport to Innsbruck Hauptbahnhof. It is straight outside the main terminal and so can’t miss it! Once at Innsbruck main station, you need to take the train to Jenbach and pick up the connection to Mayrhofen via the ZillertalBahn.
From Salzburg, you will also need to take the bus from the airport to Salzburg Hauptbahnhof. You will then need to pick up the train to Jenbach and connect as above on the Zillertalbahn.
From Munich, you will need to take the subway to Munich Ost, where you can then pick up a train to Jenbach and once again connect to Mayrhofen via the Zillertalbahn.
From Vienna, there are a number of trains that will start at Vienna International airport and will stop at Jenbach without having to change trains at Vienna’s main station. This is a longer transfer but is very easy and so if flights are cheap then worth consideration.
You can get discounted tickets if you book in advance from Salzburg, Munich and Vienna.