I can add a video to audition just fine, but when I select a clip to edit in the waveform editor, the video disappears. Do you know how I can fix this? I need to be able to continue seeing the video when I am using the waveform editor.
I'm afraid that you can't view video in Waveform view at all. You've never been able to. Even if you could, there would be no point, as Audition cannot reassemble the edits you made in the audio with the video anyway. This used to be possible (up to a point) with .avi videos, but this was dropped several versions ago; there is no video coding capability at all.
The reason for this, of course, is that Audition is part of a suite of programs that can do round-trip editing between them, and being able to encode anything in Audition is deemed to be effectively redundant.