Step one is to make sure you are in play mode and not bank mode. You do that by pushing and holding the right footswitch for a few seconds. While in this mode the middle footswitch will activate the repeat function and the left footswitch will bypass the effect. Once in this mode you tap the right footswitch at the tempo you want to set. Simply tap the right footswitch 3 or 4 times at the tempo you want. The TF will then automatically determine the tempo you used and display it in either a milliseconds value or a beats per minute number. Should be straight forward once you are in the correct mode.
Hope this helps.