Sounds like you need to look at your Safari [or other Browser] settings... and you Decompression app's settings too. Many Extensions will download as RBZ archives [just like the SketchUcation PluginStore toolset]. These are effectively a ZIP archive with a different filetype-suffix. This needs to be simply downloaded into your Downloads folder: intact with the RBZ suffix etc... Then you'll need to open SketchUp and use the SketchUp > Preferences > Extensions > Install.. button and select the RBZ archive and everything is installed for you automatically... Thereafter you should be able to use the submenu items and toolbar tools the SketchUcation PluginStore toolset has set up...