THANK YOU so much for your help! I have almost all of the plug-ins working now after changing the read-write settings.
For some reason I am still having problems with ToolsOnSurface_v1.8a.rbz and Pathcopy.rb (pathcopy is a zipped folder which I unzipped, resulting in subfolders within a pathcopy folder). When initializing SketchUp there a Load Error message: Error Loading File ZLoader__OnSurface.rb / no such file to load -- LibFredo6.rb. And for Pathcopy, it is visible, but grey and unable to be selected through Sketchup > Preferences > Extensions > Install Extensions...
I might just give up on these two. But if you have an idea I'd love to hear it!
Thanks again,
Rebecca