Also, how am I supposed to get kills with them? I shoot someone with the plasma caster but the grenade doesn't stick or kill anyone!!! I throw a sticky grenade but it just bounces off.
The Plasma Caster has two firing modes:
When firing the Plasma Caster in this mode, the shots have a fair bit of arcing, and will explode (with a fair blast radius) a short moment after they bounce off a surface, including an enemy Spartan. Use this to flush out people behind cover, and not for direct combat. It takes two hits to kill a healthy Spartan, though the blast radius of each shot can help. Treat each shot like a Frag Grenade - you can time when each shot can explode after they bounce off a surface by changing your aiming angle.
A steep angle (pointing towards the ground) will cause the shots to explode in midair. A normal angle (pointing forward) will allow the shots to travel for a bit before exploding.
After a short charge-up, the Plasma Caster will fire a stronger shot that flies much faster and straighter than the standard shots. Unlike the standard shots, these rounds will stick to anything. After sticking to something, the shot then explodes into 5 smaller submunitions in a random pattern, creating a considerable blast radius. However, there is a 3-second cooldown after firing a charged shot.
A Spartan will die instantly from a direct stick, and the shots can be stuck to any floor, wall, or ceiling. The charged shots can also mess up vehicles to some degree. Having a red crosshair will cause the shots to track enemy vehicles with a shocking degree of magnetism - if there is no obstacle in the way.
Try spawning the Plasma Caster (and its REQ variants) in Forge, and practice for a bit.