How can I play along with a MIDI sequence in Mainstage?

Hi folks,

I have some live performances where I would like to play along with MIDI sequences. I've been using Mainstage and software instruments, but can't for the life of me figure out how do play along with MIDI sequences. There's a nice plugin (whose name escapes me at the moment) that lets you play along with looped audio tracks....but a MIDI file would be better for my purposes. Is there a way to do this?

Thanks in advance,

-Mike Kaplan