In MM-15, the metronome is associated with the Vita MIDI editor.

Click on the Synthesizer tab at the bottom of the screen, then double-click on the Vita synthesizer.  This will create a MIDI object in one of the tracks and bring up the MIDI editor.  Hit the (CTRL + DELETE) keys to clear the default MIDI information.

At the bottom of the MIDI editor, click to highlight the Metronome box at the bottom of the screen.

You can hit PLAY to verify it is on, or close the MIDI editor window and hit PLAY on the arranger.  The metronome rate will be determined by the tempo of the arrangement.  You can set the tempo to whatever you desire at the bottom of the arranger window.

I highly recommend setting the metronome and track tempo before recording any other material in the arrangement.  The metronome cannot be reset separately from the arrangement tempo, and the MIDI object tempo cannot be changed using the Time Processor.