Continuing the discussion from Welcome to our forums! Please introduce yourself :
@tonychilver, your profile indicates you are using the free web version of SketchUp. If this is correct, there’s currently no implementation for extensions.
If, on the other hand, you are using SketchUp 2019 Pro (the desktop version), extensions are not opened in SketchUp. Instead they must be installed. If you’ve downloaded them to your computer, go to Window>Extension Manager and click on Install Extension. Then navigate to where you saved the extension, select it, and click Open.