I think IsKeyDown() is a Windows function to check for the computer's keys, not for the control surface...I might be wrong, let me finally check the Reaper control surface source codes more closely...
There's a bunch of VK_(KEYNAMEHERE) constants defined in Windows but I don't think you are interested in that, right?
edit : Ok, a quick look at the csurf-source codes didn't make it entirely clear how all that works...IsKeyDown() seems to be a function defined in the Cockos source code, on the other hand the VK_THEKEY-codes strongly suggest it is Windows stuff...