Just run the installer and choose the vst folder, where all your other vst plugins are installed. Then you'll be prompted to choose the location for the application folder. All libraries will be installed there, so choose a big hard drive. Reaper will recognize it automatically, if Nebula is installed where all your other vst plugins are installed too. You can always add directories in Reaper's preferences under VST, which Reaper will scan each time you open it.