The add button on the remote is on the screen in the configuration menu which is accessed through the little drop down bar at the top of the screen. normally you put the remote in find mode and then you have 30 seconds to press the button on the base station which is located right next to the screw on antenna. I would suggest having a pencil or something slender to push that button with.

But to start with you need to have both remotes assigned to the base in studio pro using the same file just add a second remote to the configuration and either program it exactly the same or make whatever variations you need to.