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Try Repairing your disk permissions. That may help:
Close all applications
Go to applications/utilities/Disk Utility
Select your Mac's volume
Click on "Repair Disk Permissions"
Restart your mac.
If that doesn't help perhaps you could post back answers to the following:
1. Are you using Logic 8.0.2?
2. What name are you giving to the mp3 and which folder are you trying to save it in?
3. If you try and save it to the desktop do you still get the same error message?
4. Does bouncing to Aiff/wave (PCM) work?