Once you select a video for trimming on the left side of the Presentation Video Trimmer window, you are presented with a trimming player on the right side. The trimming player allows you to play the video while you are trimming it and to watch a preview of how your trimmed result would look like.
On the lower part of the trimming player you can see a number of key frames from your video, with a yellow box layered over them.
The left and right edges of the yellow box are your trimming sliders. The left one marks the starting frame of your video, and the right one marks its ending frame. Drag these sliders to the exact points in the video that you would like the video to start and end.
If you own a MacBook Pro with a Touch Bar you will notice the same trimming sliders show up there, and you will be able to adjust your start and end frames similarly using touch.
To see a preview of the trimmed video, click on the Play button to the left of the trimming sliders.
When you are done dragging the slides, click on Set to record the changes for the current video that you are trimming. You will have to also click Save afterward to actually save the changes to a file.