The Medford Summer Recreation Guide on the page that tigerbright linked to says:

Parking: For all Families who do not have a Wrights Pond parking pass a shuttle bus will
be provided from 5pm to sunset. The Shuttle bus will pick up on Elm St. at two locations 1)
Flynn Rink parking lot and 2) from the MBTA #134 bus stop, at the bottom of Elm Street.

Seems a bit silly to make walk-in or bike-in folks go to a bus stop, plus I don't see any #134 stop anywhere near Elm Street.

The Summer Recreation Guide refers people to http://medfordfamilies.org/summer.php for more information on the concerts, but I don't see anything there.