The Ram ability card is found on a table next to the pool overlooking the Main Stage. You'll visit this area during the ‘Dance For Me’ DLC quest, after the Gruesome Twosome boss fight, but can return here whenever.

With the ability equipped, either train a group of at least five zombies, or use the meat bait curveball (if you have it), to force them to group up. Line up with them and use the ability by holding + and run through at least five of them. You don't have to kill them, just hit them with one use of the ability.