very easy:

Take of the frequent train services from Zurich Airport to Zurich Mainstation.
From there catch the S4 train to Saalsporthalle/Sihlcity (depart from underground track 1)

If you're going to the Sihlcity Four Point Hotel you could also consider taking the Tram 13 from Zurich Mainstation, as it stops almost directly in front of the hotel.

Trains & Trams are generally easy to handle.