The Civic Center Parking Garage, 59 Harrison Avenue, Springfield, MA, can be easily found off Exit 7 from I91. Just continue through Boland way (the road that you are on when turning left from the exit). At the second traffic light you are on Main Street, and Tower Square (where the Hackathon is) is on your left (northwest corner). Continue through the light to Harrison, and you will see the Civic Center lot on the right just before Dwight street.
Pull in on the right. There are low steel I-Beams so if you are in a high vehicle like a van this could be tricky. Park and walk out the way you came in, walk back down Harrison, and enter Tower Square in the corner. When you enter, you will pass Hot Table (good snack place, who are providing breakfast and lunch) and walk past the first set of excalators, down the corridor, to the second set. Up those, you will see UMass Center directly in front of you.
The following screenshot from Google Maps should help: