I know its on Beaver Street near Bentley (there's an old building that says "University of Massachusetts, Waltham" there which always suprised me). I think there's also some rose growing group that uses the land too.

Here's some more info w/ more pics from the city site:
http://www.waltham-community.org/CommunityFarm.html

I've never known anyone that volunteered there, but I've done similar work at a place in Lincoln and had a lot of fun :)