In the Pages pane, click a page that has a video or can hold one.
In the tool box, show the Sign Language Tool.
Click
Advanced.
Click
Import.
The Open dialog box appears so you can find the desired video.
The type is set to (and limited to) Video files (*.mp4).
Click the desired video and then click Open.
Now that video appears on the page. If there was an existing video, it was replaced on the page by the one you imported. The previous video remains in the folder.