There's nothing to choose from, as in there are no MIDI files?
If that's the case, grab the correct .ini file [img=http://forum.mixvibes.com/downloads.php?view=detail&df_id=60]here[/img]. Once downloaded, drag and drop the file into the "MIDI" folder which will be located inside the MixVibes folder on your hard drive. Restart MixVibes and this should hopefully take care of that.
Regards,
Phaded