I read on http://designreform.net/2009/04/rhino-5-beta-is-on-line/
"Rhino 4.0 Plug-ins will load and run in the 32-bit version of Rhino 5.0. Plug-ins will need to be recompiled for the 64-bit version of Rhino by the plug-in developer. Until they’re ready, you may need to run the 32- and 64-bit versions of Rhino 5.0 side by side."
Is there a version of Grasshopper that will work with Rhino 5.0 Beta?