One of the cabins around the map will have a radio tower next to it. Find that cabin and search it for a large radio on a table. Simply walk up to the radio and hold A. If the power is out for the cabin, you'll need to repair the electrical box, first.
While you will gain progress towards this achievement either way, if you want to actually summon Tommy to the match, you will want to wait until at least one counselor has been killed so that a player is ready to spawn.