Here are is the bus schedule for sunday (May 25th) evening:

Busses leave from Salzburg, main (train) station, going directly into Strobl.

The busses are labelled  "Bad Ischl" (i.e. this is what you should look for!)

 
1. Fahrt Dauer: 1 Std. 12 Min.
18:15
19:27 		ab Salzburg Hauptbahnhof
an Strobl Busbahnhof 		Postbus 3000
Richtung Bad Ischl Bf (Bus)
  2. Fahrt Dauer: 1 Std. 12 Min.
19:15
20:27 		ab Salzburg Hauptbahnhof
an Strobl Busbahnhof 		Postbus 3000
Richtung Bad Ischl Bf (Bus)
  3. Fahrt Dauer: 1 Std. 12 Min.
21:15
22:27 		ab Salzburg Hauptbahnhof
an Strobl Busbahnhof 		Postbus 3000
Richtung Bad Ischl Bf (Bus)