The free TiVo Desktop software has a hidden feature that lets you transfer videos from your Mac to your TiVo.

I have a TiVo connected to my wireless network. How can I transfer videos back and forth between my TiVo and my Mac?

If you have a network-capable TiVo (such as a TiVo Series3 or TiVo HD) that you’ve connected to your network via Ethernet or wireless, you can transfer recorded shows from your TiVo’s Now Playing List to your Mac—and, for the first time, from your Mac to your TiVo—using Toast 10 Titanium ($99, www.roxio.com). Additionally, Toast can convert those videos into a format that you can play on a large variety of video players, including the iPod, iPhone, Treo, BlackBerry, Apple TV, and PlayStation 3.

If you don’t have Toast, you can get videos from your Mac to your TiVo one other way. First, you’ll need to convert your videos into MPEG-2 format, using a utility such as VLC (free, www.videolan.org) or Compressor (part of Final Cut Studio; $1,299, www.apple.com). Your TiVo will not properly recognize any other video formats, including MPEG-1 and MPEG-4.

Then, download and install the TiVo Desktop software (free, www.tivo.com/buytivo/tivogear/software/), which lets you stream your iPhoto library and your iTunes music to your TiVo. TiVo Desktop doesn’t give you any video functionality off the bat, so you’ll need to enable its hidden video transfer feature. 
First click the Stop button in System Preferences > TiVo Desktop. Launch the Terminal (located in /Applications/Utilities/) and type: defaults write com.tivo.desktop FileVideo -dict-add VideoUIEnabled -bool true

Reopen System Preferences > TiVo Desktop, and you’ll see that a new Videos tab has appeared. Check the box to publish your videos and choose the folder that has your MPEG-2 videos in it. Note that before you click the Start button, your MPEG-2 videos must already be located in your chosen folder. If you decide to add more videos to that folder later, you’ll need to stop and start TiVo Desktop again for it to recognize your new videos.

Finally, on your TiVo, scroll all the way down to the bottom of your Now Playing List, and you’ll see your Mac listed there. Choose the videos that you want to transfer, and TiVo will transfer the movies into your Now Playing List so they’re available even after your Mac is turned off.

By the way, as soon as you turn on the TiVo Desktop system preference, it will create a properties text file for each MPEG file in your selected movies folder—you can edit this properties file in any text editor to give your video a description that shows up when you select it in your TiVo’s Now Playing List.