This is a known issue with Absynth and Digital Performer. Here is a workaround that will allow you to use the two in conjunction:

*Go to Cycling74's web page and download their free Soundflower software.

*Install Soundflower as per the instructions, and reboot.

*Launch Absynth and go into its Audio/MIDI settings and tell it to output to Soundflower (2ch).

*Launch DP and go into your hardware config. You will need to shift-click on whatever interface you normally use and then shift-click on Soundflower. DP allows you to select multiple "audio interfaces" (even though Soundflower isn't really an audio interface) at the same time. Click OK.

*Open a new aux track in your DP project, and scroll to New Stereo Bundle and you'll see a new choice: Input Stream #1-2. Choose that. Set the aux track's output to wherever you normally monitor from.

You should now be good to go! To sequence from DP, you will want to enable interapplication MIDI (create a DP Output-1 in the Setup menu->Interapp MIDI) and then set Absynth to respond to the DP Output-1 MIDI device.

Another useful tip is to go into DP's Studio menu->MIDI Patch Thru and select "patch thru in background" which will enable you to play Absynth no matter whether DP or Absynth is active.