The Stamp Student Union is right in the middle of the UMD campus. The full address is:
3972 Campus Dr, College Park, MD 20742
The easiest place to park will be Lot 1, which is free on weekends and at about a 5 minute walking distance from the venue. You can find a map of all UMD parking lots here. There are a few more lots with more space that are slightly further away. Everything in green is free to park in on weekends.