Yep, key points to get the hang of it...
Code:
if ($did == 15) && ($did(ctcpblocker,15).state == 0) {    set %ctcp_blocker.userinfo off   }


$did = Dialog ID, so we check if the dialog ID is 15. The state property which determines whether or not the check box is checked or unchecked. Where 0 means unchecked, 1 means checked.

So the above code will set the variable %ctcp_blocker.userinfo off. If and only if the checkbox isn't checked.

Hope this helps!

Come back if you need further assistance.