I just noticed on page 81 of the manual, it says MP3 read only. If there a selection for MP3 in the render dialog that would be very strange.
Also this
I don’t think MP3 encoding is an option in LE, but I think it’s included in Elements now (since that thread). You could try an Elements trial.
But if you’re just opening MP3 files to add metadata, a tag editor would be better. Wavelab would always re-encode as MP3 which is never a good idea.