Mac OS X has a system of access privileges for each user that can become skewed over time. If the file permissions are not as they should be, the operating system will think that you do not have the access privilege to write the authorization file and will not allow it to be written.
To correct the file permissions,
• Go to Applications/Utilities and open Disk Utility.
• Select the volume form which OS X boots.
• Choose Repair Disk Permissions
(Apple recommends repairing your disk permissions from your OSX boot disk's Disk Utility).
• When that process has completed, launch Digital Performer.
• When you are prompted to do so, put the DP Installer CD in the CD-ROM, enter your name and keycode, and press OK.
DP4 should now accept the keycode and launch properly.
For more information regarding Mac OS X and file permissions, check out Apple's "Troubleshooting File Permissions in OS X." docs.info.apple.com