Well, Jimer Lins has a playlist on one way to use motion capture (I haven't actually looked at the videos yet to know if they're anything close to what you're talking about), but as ASZ20 said--if having the order backwards--your best bet is probably to render your video first and then add the live actor on top of the resulting video in post-production, using whatever external program you're comfortable with. Source Filmmaker isn't like a standard video editor--think of it as being closer to creating a virtual world to record in than working with video files. While I'm sure there's some way to import your actor, but he or she would be restricted to a 2D plane, and SFM's strength lies in its ability to (relatively) easily animate in 3D.

Does SFM still say that it can't find Quicktime? If it's correctly recognized, you shouldn't have any message asking you about it on the export dialogue. This long after your original post, I'd be surprised if you haven't yet, but you have restarted your computer since installing Quicktime, right? I don't quite remember the exact process I used to fix it, but I do think restarting was what finally made it work.

I had the same problem. The best thing to do is use a batch renamer to remove the second number in ".dx80.dx90.vtx" and similar strings (in this case, resulting in ".dx80.vtx"). I am dual booting Linux and used a program there to fix it, so I can't really recommend anything for Windows, but you should probably make sure that whatever you use has regex (regular expression) support--the exact syntax will be different, but even something like "\(\.dx[89]0\)\.dx[89]0\.vtx" -> "\1.vtx" is a lot simpler than trying to select everything manually.