1. Create a new TouchCast with the "Video Import" theme
Note that video import is only supported by the newer, faster iPads. This includes iPad 4, iPad Air, and iPad Mini current generation. Also note that you can only import TouchCasts without vApps.
2. Under the "TouchCast" tab, tap the TouchCast that you'd like to trim to import it into a new TouchCast
3. After your imported TouchCast appears in your NEW TouchCast, you will re-record it to trim
You can control the re-recording of your imported video in the "Video Import" tab on the bottom left. When the "Video Import" tab is selected you will be able to view the video time scroll, which will help you determine when to STOP the recording, trimming the un-recorded section of the video.