If that version of the plug-in (32-bit vs 64-bit) supports that version of CS5 (32-bit vs 64-bit), then simple re-install of the plug-in should make it available to that version of CS5.
Thanks for the advise. I figured out another way to make it work. I copied the NI file from the Plugins folder in CS4 and pasted it into the CS5 Plugin folder. Works great!