So... uhh... Stupid question here...
I got the Stone Dog Guided Missile System, and I don't know how to use it. I've checked the manual, done some web searching, but I can't figure out how to lock on to a target.

So... how do you lock on to a target?

All you need to do is aim at an enemy. A red box should appear around the enemy being targeted, and a sound should play.

Lock-on is instant and lasts until one of the following happens:
• enemy goes off-screen (as in completely out of view - enemies behind objects or walls remain targeted which allows you to duck around a corner and keep hitting them with missiles)
• enemy is destroyed
• you aim at another enemy

Missiles turn towards the target after flying a few metres in the direction you're aiming when you fire.

If you've got a very high resolution (higher than 1280x960, probably) selected in the in-game graphics settings the crosshair offset bug might be breaking the lock-on system.

Ah, the offset was the problem. Read your solution and it worked. Thank you!

You're welcome, glad it helped :)