On Windows and Mac OS X, Audacity is a 32bit application so won't see 64bit versions of VST plugins, even on 64bit operating systems.; On GNU/Linux, 32bit systems will have 32bit versions of Audacity which won't see 64bit VST plugins. 64bit systems will have 64bit versions of Audacity which will see 64bit VST plugins but won't see 32bit plugins.