Open Reaper and go open the Preferences window found under the "Options" drop down tab. Click "VST" on the side menu and then click "Re-scan".
WINDOWS users will need to add the following default plugin paths-->
64bit Reaper: C:\Program Files\Vstplugins
34bit Reaper: C:\Program Files (x86)\Vstplugins
1) Ensure the plugin is installed on your system.
2) Ensure the iLok License Manager app is up-to-date on your system.
3) In some cases, you may need to hit "Clear cache". This will force Reaper to rescan all plugins.
If issues persist, please contact support here.