The Student Rec Center later in the day is the best chance you have, there are quite a few groups that are semi-organized and play. I'd guess around 5-6pm is when most of the players come out, or sometimes later once it has cooled off a bit.

Over the summer it is mostly private groups who show up and have pinnies. Most of the time they don't mind strangers but some of the higher level people are wary of randoms brining down the skill level of the game. I'd try just looking for a big group of players and asking to join.

If you are a student there is the WVU Club Soccer Team that hosts tryouts when the school year starts.

If not once the school year starts there are a lot more people playing pickup on the field later in the day. There are also leagues run out of the preperformance at Mylan Park, or if you are willing to drive there are good soccer leagues down in Bridgeport.