After opening a presentation file for trimming, you will be presented with a video list and a trimming player.
The video list, which is on the left side of the Presentation Video Trimmer window, contains all of the videos that the app could find inside of your presentation file. The videos are sorted by slide and then by shape order.
Each video is represented by its thumbnail and by the number of the slide to which it belongs.
Clicking a video in the video list will select it for trimming and start a trimming session in the trimming player.