Yes, are your running Windows 7? It seems that tivo files can't natively be formatted to dvr-ms. I doubt it will work but maybe you can try changing the .tivo extension to .mpg and then trying to convert to dvr-ms.
There's another possibility that this won't work. Sometimes, cable companies set the broadcast flag to Never Copy by mistake. Are other tivo files working, just some or none?
Lastly, VideoReDo is supposed to work with .tivo files only if there are not copy protected. I can't try as I don't have a tivo but do have a cablecard tuner and record using Windows Media Center and some files with DRM, I can't do anything with at all.