Metronome use on stage

Sure, but you need an audio interface with 2 outputs: one for the headphones, one for the speakers. MainStage can deal with multi-channel interfaces just fine and you can route the metronome to one specific channel.

Pan for a Metronome???? The rest can be set up in the preferences.

What's wrong with wanting to pan a metronome sound? I can easily imagine wanting the metronome to be heard in only one ear while having some other cues (perhaps even verbal ones) in the other ear?

There is no up-beat.

I don't know about the metronome, but if it were me, I'd probably record a click track and run it as an audio file in playback. That way I could pan it wherever I wanted, put in whatever accents/up/down beats I wanted, and route it wherever.

Hope that helps, although I have rather limited knowledge and I'm probably missing something obvious here.

Another suggestion from a different thread is to use Ultrabeat instead for your metronome. Then it will follow the tempo appropriately and you can even subdivide your beats. You can pan Ultrabeat (or just insert it as a mono instrument, which will reduce the cpu load).