Are you using V5 64 bits? I've had no problems running Grasshoper in V5 x64, but c++ plugins wont work unless they release a 64 bit version, like Brazil has. I think grasshopper works fine because it runs on the dotnet platform, but this is just a guess. Have you tried updating to the latest donet framework?
I just installed the latest WIP for V5 and all my plugins work now. Maybe it was the order in which I installed things. Perhaps I should have installed V4 first, got my plugins installed and working, then installed V5.
I don't know if that would have fixed it, but it's fine now! And I am very happy!
Thanks for the reply.