To remove Digital Performer 6's Authorization file under Tiger, go to Applications> Utilities and open the Terminal.
In the Terminal type cd /

then hit return.

Next type: ls

then hit return.

And then cut and paste the following text into the Terminal:

rm Auth-DigitalPerformer\ 6.0

Your Digital Performer 6 Authorization file should now be removed.
You should repair disk permissions at this point.
Go to Applications> Utilities> Disk Utility. Choose First Aid then highlight the hard drive that your operating system is installed on. Click Repair disk permissions. Restart your computer once this is complete.
Once your computer has restarted run the Digital Performer 6 installation disk again. Enter your name and keycode when prompted.