From Germany
On the motorway Munich-Salzburg to the motorway-triangle Inntal, to Kufstein, then via Kitzbühel, Paß Thurn, Mittersill to Zell am See.
Or you use the motorway Munich-Salzburg until the exit Inzell, then via Lofer, Saalfelden to Zell am See. This way you do not have to buy the Austrian Motorway Toll-Vignette.

From Vienna
If you do not want to come via the German Corner, use the Tauern Motorway until Bischofshofen, then via St. Johann, Schwarzach until Zell.

From Switzerland
Use the Inntal Motorway Innsbruck-Salzburg until Wörgl-East, then via Kitzbühel, the Paß Thurn and Mittersil to Zell.
Using Austrian motorways requires the purchase of the Austrian Motorway Toll-Vignette.