This may be a well-known issue, but I just set up Tivo Transfer for the first time, had a problem, and realised why. This may be of benefit to other users.
I had correctly linked my TivoHD with my MacBook using the media key, and the Tivo showed up in the device list. I saw some new recordings, and began a transfer. The animated icon next to "Active Transfers" appeared for a second, then stopped. Didn't seem to want to transfer, and no error or explanation why.
Then I checked the downloads location, and it was to the default "~/Documents/Tivo Recordings". This folder did not exist. I changed the location to my existing Movies folder, and started a transfer - which worked fine.
Hope that info helps someone else out.
Matt