I don't have my AT F with me at the moment, but if memory serves, the clips open at the top. So you need to position the wire against the steel plate, so that as you push it down onto the stem, the wire slides toward you and under the clips.
Edit: One trick I use is to get the stem lined up with the spring, and then tilt the enter away from you. Then use some type of thin implement to hold the wire down on the plate so that it guides into the clips.