Hello Nic,
I gather that you have managed to solve the problem, but for the benefit of others coming across this thread, I’ll post our initial response to this kind of issue:
Assuming you have successfully run the Shaderlight installer and you still don’t see an Extensions menu, or there is no Shaderlight for SketchUp item under it, you will need to tell SketchUp to allow unrestricted access for extensions. You can do this by choosing the Window -> Extension Manager menu option, clicking the cog icon in the top-right corner of the Extension Manager window and then selecting ‘Unrestricted’ in the Settings dialog that appears.
If you have any other tips to add, please do let us know.
Regards,
Shaderlight support