From Central Square Red Line T-Stop:
Take Western Ave towards the river .3 miles, turn left onto Gilmore or Montague streets

From Mass Pike or Storrow Drive or Memorial Drive:
Take River Street (one way) towards Central Square
Left onto Howard Street
Left onto Western Ave
Left onto Gilmore or Montague

OR
Take River Street (one way) towards Central Square
Left onto Putnam Right onto Ballord Place (just before Western Ave)

Even though it's right between major streets in Cambridge (Western and River), beware, you can't see if from those streets. The easiest way to get there -- from Memorial Drive, turn east onto River St (left turn if you're coming from Harvard and the burbs, right turn if you're coming from MIT/Boston). Going East on River St., take your 2nd left onto Putnam. Then, take your next right onto a tiny street, which will take you to the park.

Alternatively, if you're coming from Mass Ave, head South on Prospect (towards the River). Stay towards the right on Prospect, which will become Western Ave. Go 7 or so blocks on Western, past Jay St. and Howard St. At the next small street, Gillmore or Bancroft, go left. That will take you to the park. If you miss these streets and hit Putman, go left and take your first left to get to the park. If you hit Memorial Drive, you've gone too far.