Oh wait, yeah, if you're trying to kill somebody on an emplacement you'll need to pull out the knife (6 on the keyboard, 5 is the grenade), adjust yourself, and then left click. When killing mobile targets just use the melee button. It's much faster than changing to the knife and then attacking. I've pressed the melee key about 2 meters behind an enemy and sliced them up. Your character can lunge farther than it looks like he can. Even if you think you won't make it, try it anyways. Sometimes I'm surprised that I managed to get the knife kill from long range.
