Grenades are considered "powers" or "abilities" in MP as SP. and should be showing up as one of the three powers allotted (usually third) on the HUD (top of screen in MP). Best of luck trying to keep them replenished, though. Even if yours is the only character in the match using grenades, you'll be lucky to come up with one at an ammo box when you need a refill. Prepare to spend some ammo packs.
The problem is that they aren't showing up though. I have no idea why, I've been through all the options menus and see nothing that would even suggest that they can be turned on or off. Really stumped here.
