we'll need to take a good long look at Rhino5 and 64bit support when I'm in Seattle next month. There seem to be a load of problems associated with these platforms. I don't have a computer here though that can run 64-bit Rhino5 or rhino on parallels/bootcamp so it's very difficult to debug.
Two things:
Despite the error message that I keep receiving, if I open Grasshopper, I appear to have this version. I feel foolish for all my posting over the error. GH seems to be working fine.
Not that it's urgent, but the RhinoCommon.dll is an older version than the 9/29/10 version that ships with Rhino 5. I just found this out will trying to make a .gha component that depended on the most recent version.