Search Google for TMK firmware and you should get a link to github with the source code, and all the instructions you need for building, assembling and loading the firmware onto your Teensy.
The Teensy is used so as to provide a controller for a board that doesn't have one, or has a broken one. It can also be used for converting old connector types over to usb such as ADB, 5 Pin DIN, etc etc. Perhaps other Geekhackers can chime in, but I'm unsure if the TMK firmware supports usb to usb conversion.