This is what I did:
Downloaded the file from this website;
It was a zip file so I extracted it on my desktop and I tried follow instructios on Sketchup Help how to installe ruby skript.
This is what my plugin folder looks like:

Hi TIG

Yes it is comletely separate.

Hi Chris

I've the same problem as other people described. Cannot select line parallel to red axis. In my Ruby console I get Error: #TypeError: No implicit conversion from nil to integer>. Do you know what might be wrong?