Once you’ve downloaded an extension, it’s packaged as an .rbz file. To install the extension in SketchUp, follow these steps:
Open the Extension Manager from the Extensions menu or using its toolbar icon ().
Click Install Extension.
Browse your local files and select the .rbz file you want to install. Click Open.
For third party extensions, a warning will appear advising you that you should only install extensions from trusted sources. Click Yes on this message if you trust the extension and finish the installation.