I had a bit of trouble installing the plugin with multiple shared plugin paths so I thought I'd share my solution.
For some reason using local file paths on my computer just would not work in the "maya.env" file.
ie: "C:/Program Files/pmG/messiahstudio1.5/messiah"
So I had to share this folder and use UNC paths. So my 'maya.env' file in the "my documents/maya/5.0" folder contains the following line.
MAYA_PLUG_IN_PATH = \\YourComputer\messiah
If you have multiple shared plugin folders, say a whole heap of plugins on your server. Then you can use a semi colon to seperate the plugin paths but you may NOT have multiple MAYA_PLUG_IN_PATH variables.
So this works...
MAYA_PLUG_IN_PATH = \\YourComputer\messiah;\\Server\OthersharedPlugins