The DP3 User guide contains a section on Reason and Rewire on page 915-918.

1. Always launch Digital Performer first.
2. In DP's FILE menu, choose Preferences. Make sure that the box next to "Play in background" is checked ON.
3. Open a project in Digital Performer.

4. Launch Reason. Verify that Reason is in "Rewire Slave Mode". This indicator will appear at the top of the reason window, in the AUDIO OUT section.

5. Leaving Reason open, click on DP.
6. Create an AUX track in DP, set it's input to Reason Mix L/P, and it's output to your main outputs. (See page 160 of the DP3 user guide for more on Aux tracks).

Pressing PLAY on either DP or Reason should make both play. Reason's audio will be routed through Digital Performer.