Make sure you have network applications enabled on the TiVo. After that, once you start Mac2Tivo and its server your Mac will appear in the "Now Playing" list. It will not show up in the same place as the network applications do.
A couple of things. The machine and the TiVo must be in the same networking zone. If you have computers across different routers that define their own sub-networks and are simplying just extending the network. Then you tivo and your Mac will not see each other.