How to set up Reverse Kinematics?

I’ve been searching around youtube and the dev forum for the past couple hours, but I can’t fine anything that properly shows me how to set up Reverse Kinematics.

I get this screen to pop up, but nothing explains what to do from here, or how to “Enable IK”

Some things I’ve looked at are here:
And here: New to Animation Editor: Inverse Kinematics
All it shows on the later link is to click “Apply IK”, which I can’t get to show up.

So could anyone tell me how to set this up? Thanks!
Edit: I’m using a R15 model.

There should be an “Enable IK” button at the bottom of that window:

Well, aren’t I stupid, it opened up in a way I couldn’t see that. Thanks!