After saving your presentation in PowerPoint, launch Presentation Video Trimmer to trim the videos that are in the saved file.
You can start by clicking the Select and Trim button, clicking the same in the File menu, or by typing ⌘O on your keyboard. These will all bring up a file opening dialog which will allow you to select the presentation file that you would like to work on. Alternatively, you can simply drag a file from Finder to the Presentation Video Trimmer window. You can see what file types can be opened by visiting Supported File Types.
Once you select or drag a presentation file, the app will open and scan it for videos. When it is done, it will present all available videos arranged by slide they appear on.