I haven't got CS5, but on CS4 the 32-bit version of Photoshop installs into program files(x86)/adobe/adobe photoshop cs4, and the 64-bit version goes into program files/adobe/adobe photoshop cs4 (64 bit). In each case the plugins go into the plug-ins folder of that directory. They don't (at least on my system) go into the common files folder.

But if you do put them there, I guess it would be program files(x86)/common files/adobe/plug-ins/cs5 for the 32-bit version and the equivalent folder in the program files/common files hierarchy for the 64-bit version.

Steve