In the skills menu, go to the force (magic) abilities. then go to force abilities. It's the fourth ability down. You need a minimum level and force mastery level of 17 to get it. Its picture looks like this:
And as was said before, to use it on someone, you need to be holding down shift as you click on them. As for help, if the in-game guide (hotkey H) was no good to you, the guides in this site are pretty easy to find and understand. Just look at the stickied posts.