Here's the original PDF manual for the D-50 (linked from this page) - http://users.encounters.be/D50VE/guide.html
I have just gotten my third D-50 (also had a D-550 once and I've also had the V-Synth with the D-50 expansion) and I've found much joy in creating new sounds on it. However, having it laid out on a computer screen would be funky. I've not seen a D-50 editor since I saw Steinberg's old editor for Atari... or was it Geerdes?
By the way, I sell sound banks for Reason and some hardware synths on my webpage - synth programming is what it's all about!
Best,
Patrick Fridh aka Bitley™
http://bitley.laconicsounds.net