Try the latest version of Thomthom's "Simple Plugins Installer", and after you've installed that and restarted Sketchup, you can try to reinstall the RBZ archive using it, rather than the native-installer [which I assume you have used initially]... It should spot some permission/VirtualStore issues that the native-installer might miss... It might still be an 'security/permissions' issue... Have you checked this, as I suggested previously ? The Plugins folder's security/permissions needs to be FULL for you [and All] and its contents/subfolders...