Maybe it would be a good idea to let SD3 create the folder automatically when installing if it’s missing. After some research I could finally solve this exact problem, but it took some time and I’m sure there are plenty of users out there having the same issue with the missing folder.
For me the user midi function was the final piece for this astoundind plugin, but I think this issue, as small as it is, should not appear for the sake of satisfaction 
. Maybe you can add some sort of automatic folder creation in the next update!
Just my two cents 
I had the same issue and solved it about 5 minutes ago thanks to your tips! Maybe it’s helpful if I tell you what exactly was the problem:
Thanks to your postings I searched C: for the errlog.doc where I could find the error “ERROR opening file C:/Users/wolfg/Documents/Toontrack/EZdrummer/User MIDI/Verse0.mid”. The real problem was that there was no folder with this name in this directory!
So I just went to C:/Users/wolfg/Documents/Toontrack/EZdrummer and created a folder named “User MIDI”. Then I started Superior Drummer and NOW I CAN FINALLY USE THE USER MIDI FUNCTION!!! 
I’m really happy right now — thanks a lot to you guys! This was the missing piece for an astounding software!