I think you'll get some benefit from the chapter "The Basics" from my book:
http://www.bryanray.name/wordpress/blac ... he-basics/The UI won't match what you see because I started writing way back in Fusion 7 (!?), but the essentials haven't changed at all—just the appearance.
Maybe read the sections "Lesson the First: Merges" and "Lesson the Fourth: Masking" first to solve your immediate problem, then go back and check out the entire article.
You'll just need to replace the Polygon I demonstrate with your roto. Sander's description was a little vague, so I'll explain the Bitmap in a bit more detail:
Bitmap has two inputs: Anything you put into the yellow input will have one or more of its channels converted into a single-channel alpha suitable for use in any mask input. Put the output of your roto's Loader or MediaIn into that yellow input.
In the Inspector, choose the channel you want to use in the Channel control. If your roto is a typical RGB image, the output of the Bitmap will probably be solid white because no alpha is usually interpreted as alpha = 1. If that's the case, change the Channel control to the channel you wish to pull the mask from. For a black-and-white mask image, you can usually use any channel, or Luminance.
Here are three ways of applying a mask (these aren't the only methods, but they're the most common):
![Capture.JPG](./download/file.php?id=46674&sid=b2fb6c916f8f3ac051025d0de1c9db85)
- Capture.JPG (112.46 KiB) Viewed 1396 times
The blue input on every node is intended to take masks, as described in the article linked above.