Hi,
I have exactly the same issue as Gonz above.
Purchased MP3 upgrade, successfully installed - license OK, go to File, Save As MPEG-1 Layer 3 MP3 (which wasn’t an option I had before) - OK, but then on the dialog window behind this which is now revealed, the Output Format is what it was before (in my case OSQ) and selecting the drop down list Factory Presets, etc. does not list an MP3 format.
I did notice the MP3 installer installed a 32 and 64-bit version to the C: drive which I had no control over, yet my WaveLab LE 8 I installed on D: 