Grenades in WFC

Alright, I finally (FINALLY) got War for Cybertron for the PC. It's been almost 2 weeks since i bought the game and I'm already at Death of Hope (I've only played for aboiut 10 hours, not all together). More to the point, I can't figure out how to use grenades of any sort. How do they detonate? And I can't use the magma-frag launcher at all. It says RBM or something like that to denonate, but I have no idea what that could possibly mean. Does anyone playing PC know how to use Grenades. Help is greatly appreciated.