As long as you install the plugins and scripts into directories that Maya usually looks for them you don't have to edit the environment variable, but if you are using a directory outside of where Maya usually looks for them you need to enter them under the path variables.

I'm not gonna be home till the end of next week so I can't work with Liquid here. Now if you have to install the scripts into a different directory than Maya usually looks for them you have to edit that environment variable. You do that by opening the variable to edit, then add ";" (no quotes) followed by the path needed. That will tack on after the Renderman script path. I would try installing those scripts into a normal script directory first before editing the variable....less mess and fuss

Once you get this loaded you'll have to let me know how you like it

Darkon

P.S. If you screw up the path, you can always just delete the new part of the path and go back to the original by deleting from the ";" and on