First, there's no point in opening .rbz files when you download them. Use Install Extension in SketchUp's Extension Manager window to install them. Better yet, download the SketchuCation Extension Store tool from Resources at the top of this page and install it using Install Extension. Then use the Extension Store tool to find and install extensions for you. Doing it that way will help you keep the extensions up to date and any extensions you install with it get saved into a bundle which makes them easier to install on another machine or in a new version when you upgrade.