To summarize, this is the step-by-step procedure that I have followed in order to successfully install mp3PRO on QMP Build 104 (the latest version at the time I am writing):

1. Uninstall any previous version of QMP or QCD. 2. Install QCD version 4.50 from the Version History page (scroll down in the QCD Version History window until you find it). 3. Uninstall QCD version 4.50, but, when asked, leave the plug-ins and and Cddb controls installed. 4. Restart the PC. 5. Install QMP Build 104 (from the same Version History page) in the same directory where QCD version 4.50 was previously installed. 6. Start QMP. 7. In the list of Input plug-ins in the Preferences, make sure that the Quinnware mp3PRO plug-in is enabled, and move it up above the MP3 plug-in, or disable the MP3 plug-in. 8. Close QMP, start it again, and enjoy!