Does the PopcornFX plug-in show as being installed in the Reallusion Hub? If not it may not have been installed yet. A common problem is that PopcornFX is made up of various content packs and the plug-in itself. If you have only installed the content packs like PopcornFX Library 40 then you won't have access to the plug-in features.
If the PopcornFX plug-in is showing as installed and it is still not appearing in iClone even after a restart, then please uninstall the PopcornFX plug-in and re-install again. Importantly do make sure iClone is closed down during the install or the plug-in may still fail to appear in the plug-ins list.