Peavey Revolver 4, being an older outdated VST will only run in 32-bit mode on a Mac. It will run in 32/64 on a Windows machine. You need to make sure that you have installed the 64-bit version of Studio One if you wish to use the 64-bit version of the plug-in. On a Mac, you will want to right-click (ctrl-click) the Studio One app in the Applications folder and go to "Get Info." You can then check the box to run the app in 32-bit mode. To enable a VST plug-in in Studio One, just go to Studio One>Options (Preferences on a Mac)>Locations>VST Plug-ins and set the location that you have installed the VST plug-in to.