The deprtment maintains a list of maps and directions with more options than offered below.
This map is a few years old, but should work for you (look out for extra buildings that have popped up in the meanwhile). Wean Hall is building #2, labeled "SCS". If you're coming by car, visitor parking is available at the corner of Forbes and Beeler (where you will find a large parking garage, not shown on this map). You can find metered parking along Frew Street, but it's usually filled up unless you get here real early (~8am).
If you're walking along Forbes from Oakland (where the hotels are located) cut into campus at the first opportunity, through parking lot 15, and follow the human flow, which should take you to the entrance to Newell & Simon Hall (more or less building 20 on the map below). You will enter on the 3rd floor; go up to the 4th floor and proceed over the bridge to Wean Hall, 4th floor. This is probably the least complicated way to get here.
The main pedestrian entrance is at Morewood and Forbes, where you will also find a campus map. It's a small enough campus that just about anyone should be able to direct you if you feel lost.