This is probably the result of an incompatible or corrupted mp3.dll file. Follow these steps to replace it with a fresh copy.1) Quit Pro Tools. Make sure you have the installer CD for the version of Pro Tools currently in use.2) Open Macintosh HD > System Folder > DAE Folder > Codecs > MP3. Drag the mp3.dll into the trash. Leave the MP3 folder open.3) Open Pro Tools Installer CD > DAE Folder > Codecs > MP3. Drag the mp3.dll you see there into the MP3 folder residing on the Macintosh HD.4) Launch Pro Tools, you should be able to use MP3 Option free of errors now.