Ricky,
I heard that Mentor Graphics Expedition may have either included DDE as a subset, or just integrated some of its features. If they did indeed integrate it as a subset of Expedition, maybe you can sign up to do a trial and you could possibly get all the documentation. Maybe an Expedition user can comment on this? I don't know the program, but just heard from a coworker who used in his last job.
I no longer have any DDE documentation, but I heard that there is a place in Denmark that somehow still sells it. I put some inquiries out there to find more and hope to have good news for you