Another possible tip for Snow Leopard users.
I'm still trying to resolve the aspect ratio problem, but when I tried to copy some sample movies from my MacBook Pro to my TiVo, no files showed up (again). Rather than do a complete uninstall and reinstall of Toast, I simply force quit the Mac2Tivo program and then went into the Activity Monitor and force quit a process called RXTivoServer. Then, when I relaunched Mac2TiVo, it worked fine.
Regarding the aspect ratio issue, Mac2TiVo only converts videos to MPEG2 on the fly and doesn't change the aspect ratio. So far, all of the lo-res movies that I've uploaded to my TiVo are up-res'd to 720 x 480, which is an aspect ratio of 3x2 or 1.5. Therefore, on a widescreen 16x9 (1.78) display, they all appear slightly letterboxed and stretched horizontally. That, to me, appears to be the issue, but I think it might be a TiVo issue, not a Roxio issue. An aspect ratio of 1.5??? Whose idea that that?
Roxio's last suggestion was converting everything to MPEG2 before transferring, but I haven't had time to try that yet. I'm going to try to attach a PDF of examples that may be useful to those experiencing the same problem.