The Macintosh version of TiVo Desktop officially doesn't support the publishing of videos back to the TiVo. However, there is a "hidden" videos tab that can be exposed by holding down the apple/command key when you click on the preference pane.
However, the metadata format changed slightly in .tivo files with the 9.1 software release -- and this unofficial feature no longer works with these new .tivo files. However, it does still work with MPEG-2 files (same as the non-Plus version of TiVo Desktop for Windows)
For other file formats, I personally use TiVo.NET (TiVo Dot Net) on my Macintosh machines which has been working for me for well over a year now (and under Leopard too). It now appears to be abandoned by the developer though. There's a thread here on it -- and A@ron created a preference pane to go along with it.
TiVo.NET supports well over a dozen different file formats including MP4 and AVI
You can never have too many TiVo's (7) or wireless routers (5) -- all in one residence!