1) You will need to have FreeMIDI 1.35 (or newer) installed. In the FreeMIDI preferences, select Emulate OMS, Inter-Application MIDI, and FreeMIDI Applications Only.
2) Open FreeMIDI, and while FreeMIDI is still open, launch Rebirth, and then Performer.
3) Open the Transmit Sync window from the Basics menu and select Rebirth from the Applications option in the MIDI beat clock menu.
4) Quit Rebirth, and re-launch it.
5) Select Digital Performer (or Performer) for the sync input setting in the ReBirth Preferences. These are located under the edit menu.
6) Set ReBirth to External MIDI clock.
Pressing play in Performer will now start ReBirth playing in the background.