In Chasm City, pioneer Marcos Ferris says that the ancient alien Grub Gideon brought the Melding Plague to Yellowstone after getting trapped while hiding from the Inhibitors. Ferris goes on to explain the Plague's origins in this Chasm City passage:
"Did he[Gideon] know he had it?" I asked.
"I very much doubt it. The
plague was probably something he carried without even knowing it; an
old infection to which he had long since adapted"
From Races in Revelation Space in the Grubs subsection:
Another grub crash-landed on Yellowstone millions of years ago,and was still alive during the events of the novel Chasm City (though it was dead by the time of Redemption Ark). Examining the wreckage of this second grub's ship helped humans to recreate its inertia-suppression technology. It was this grub that brought the Melding Plague to Yellowstone. The grubs appear to be immune to the plague, probably because of the plague-preventative agent that is produced by them. This liquid was concentrated and distilled into a form that was agreeable with the human metabolism, and was one way of protecting plague-susceptible implants and nanotechnology.
It seems like the Melding Plague's origins are wrapped up with the origins of the Grubs. The people on this forum thread The Melding Plague (Alastair Reynolds) Possible spoilers seem to be of the same opinion.
While I have only read Revelation Space, from what I gather there is no explicit statement about the precise time and place where the virus was created. Much like in Day of the Triffids and some other Post-Apocalyptic stories the reason behind the apocalypse is kept intentionally obscure so that it is all the more terrifying and curiosity-inspiring. Also it allows authors to focus on the visceral day-to-day lives of the characters rather than the conditions that put them there.