First you should note that MC10 is required for Windows Media stream playback via the TiVo HMO. If you already have MC10, read on. I also assume that you already have Media Server up and streaming away to your TiVo.
1. Create a playlist and call it "Radio" or whatever you want.
2. Open an MP3 or WMA stream in MC.
There are a couple of ways to do this. If you have the URL already, go to File > Open URL and enter the URL there, or go to Web Media in the navigation tree, find a radio station in the right format (must be MP3 or Windows Media), and click the playback icon.
3. Once the stream is playing back, click "Playing Now" in the navigation tree.
4. In the bottom right pane, locate the row corresponding to the currently playing stream. (You can type in a friendly name for it in the "Name" column if you want.)
5. Right-click the row, and from the menu, select "Send To" and then "Playlist" and then the name of the playlist you created in step 1. (Alternatively, you could drag-and-drop the row to the playlist in the Navigation tree if you have the Playlist branch of the tree expanded and visible.)
6. To add more streams, repeat 1-5 for other stations as necessary. For adding multiple streams to the playlist, you could also, I suppose, keep adding streams to the Playing now list, and then select all the rows and then follow step 5.
7. You should now be able to open the stream(s) in the TiVo HMO by navigating to the Music section, and then going to Playlists > {Name of your radio playlist} > {Name of your station} and pressing Select or Play on your TiVo remote.
Note that there is a known issue in MC10 with playing back some Windows Media ASX audio streams in that MC incorrectly identifies them as WMV video streams and thus won't stream them to the TiVo. But as they say, "Your mileage may vary."