I'm trying to add a mask to my clip in the VSE but the menu stays empty.
Am I doing something wrong?
The first thing that you have to do is create a mask.
There are two places where you can do this:
In the Image Editor.
Or it can be created on the Movie Clip Editor
Go to the Motion Tracking Screen Layout and load in your movie. Select Mask in the bottom options panel where it probably currently says Tracking.
Key Ctrl + LMB to add a new control point. Keep clicking to mask your object. When done, key A once or twice until everything is selected and key Alt + C last to connect the polygon. If you want the mask to move, key I to insert a keyframe, move the time cursor in the Timeline, move the control points, and key I again. They will glide in between.
When you are done with the mask, go back to the Video Sequence Editor, and your mask should be there.