I also encountered this problem with my recent Grasshopper installation on a Windows 7 SP1 x64 and having both the 32-bit and 64-bit version of Rhino v5. It seems double-clicking on the RHI file would install Grasshopper for the 64-bit Rhino and the 32-bit Rhino cannot see the installation although it was successfully installed.
My observation is that both Rhino Installer Engine executables (x32 and x64) are named the same way and that Windows Explorer would associate the RHI file with just one (preferably x64?).
What I did to get 32-bit Rhino to recognize the Grasshopper installation was to drag the RHI file to the 32-bit rhiexec.exe (located in C:\Program Files\Rhinoceros 5.0 (64-bit)\System\x86 in my case). Now both versions of Rhino can open the Grasshopper installation. I hope this helps anyone else.