At First you have to find the backpack from Max, that is between the pinboard and the bookshelf. Then you have to open it in your inventory and you'll find a book about australia inside, you have to take a look closer and then you find 4 flags. When you take a look around the bedroom from max you see a map from australia at the wall, you have to click on it.
You have now to put the flags in the 4 Towns on the Map, which are missing in the list of contents in the book. A secret drawer then opens under Bed from Max Gruber, in this are the keys for the Museum in Berlin and for his Office.
The 4 Towns are the follow Edit