Howdy! Been using Reaper for about a year now and LOVE IT (I'm an Acid Pro refugee) and haven't had many problems that a little sleuthing here couldn't fix - until now, that is.
DSK* products are notoriously CPU-heavy sometimes (and often have complaints about GUIs and runtime bugs and whatnot) but as they say, for the money their stuff is decent. As such though this issue may or may not be for the Cockos crew...
However, I just downloaded two newer DSK freeware plugs (Overture and Guitar Steel) and the FX dialog doesn't even see 'em. I cleared the FX cache and re-scanned everything, and Overture *did* turn up on the list of plugs it sees when initializing, yet it's not there when I go to add it.
* DSK website:
http://www.dskmusic.com
Win2k/Reaper 3.72