Hey All,
Tangier, you're confusing the regular shape-drawing tools with the shape-MASK drawing tools. You can draw an arbitrary mask directly onto any object in Motion, including video.
Patrick pointed out that you could draw a shape and then use it as the source for an Image Mask (different from a shape-mask), but in your case, just draw yourself a shape-mask using the rectangle mask tool in the toolbar. Like any other shape-type object, you can feather the mask as much as you'd like.
It's really amazing what a particular **** it is to try to explain to someone in written words that Motion has shapes, shape masks, AND image masks that can use shapes as the mask input. It's one of those things that takes only a moment in person, but paragraphs when written.
That being said, the second page of the huge masks chapter in the manual explains it beautifully (HINT, HINT).