First, you'll need to go to the rhythm guide and create a rhythm arrangement. On measure 1, select the last rhythm pattern available (Metro), and set the length for the length of the piece (or more).

Then, make sure the Rhythm Arrange light is blinking (this will turn the metronome pattern on or off) and make sure the fader is up.

The metronome should be now be available for recording and/or playback.