There are two ways to do this:
1. Go to the mentioned menu (AUX SW). The cursor arrow on the display should be pointing to the left, enabling you to choose the parameter you want to assign your aux switch to with the encoder (i.e. bank up, down, tap etc.).
Select TAP with the encoder. Press the right footswitch of the TimeFactor to toggle the arrow on the display to the right, enabling you to choose tip, ring or both. Select tip, ring or both via the encoder (depending on your external footswitch's button assignment- if you don't know about this, go to 2.). Leave system mode by holding the encoder button and the right footswitch for a few seconds (same way as entering it).
2. Do the same as in 1. up to the point where you changed the arrow's direction (should be pointing to the right now).
Instead of selecting tip, ring or both, you press the right footswitch on the TF for a second time (after you did so to get the cursor arrow on the display pointing to the right). Display shows "LEARN" now. Press a button on YOUR external footswitch and the TF will recognize its assignment (tip, ring or both) automatically. Leave system mode.
This is a great feature which also works for MIDI messages. And it's described in the manual :))
I hope I haven't misinterpreted your question.
Good luck.
