I had to put the remote into the IR mode than I was able to learn the code into the MX780.
Switching to IR mode
To enter IR mode, press and hold the TiVo+Red C buttons until the activity indicator on the remote control lights. The remote remains in IR mode until you enter the key combination that puts the remote into RF mode.
Switching to RF mode
Entering RF mode causes a paired remote that is currently operating in IR mode to switch to RF mode. This procedure has no effect in New Pairing mode or when the remote is unpaired. To change the remote control from IR mode to RF mode, press and hold the TiVo+Green D buttons until the activity indicator on the remote control lights.