Important: The GM Tool needs Java 1.5 (or newer) - you have to install the apropriate JRE (Java Runtime Environment) for your system. See java.sun.com for details and downloads.
GM Tool is a simple executable Java .jar file. Simply unzip it somwhere (including the /libs directory) and you can start it, either by double clicking the “GMTool.jar” file (in most graphical file managers) or by typing “java -jar GMTool.jar” in the console. As long as Java 1.5 is installed on your system and the /libs directory is present, this tool should work on any system.
To add Plugins, simply create a subdirectory /plugins and copy the plugin .jar file (or directory-structure) into this directory. Then you should be able to choose it in the “Options” tab.
To add your own tokens, create a subdirectory /tokens and copy the tokens (.gif) into this directory. The token names should appear in the right list in the “Pictures” panel as soon as you restart the tool.
The only problem I'm aware about at the moment is, that WinAce (and perhaps other compressing programs) seem to sometimes want to open .jar files themselves, preventing them getting started by Java. If you double-click on the .jar and WinAce, WinRAR or something similar opens, then you'll probably have to right-click on the file and use “Open With…” to select the Java Executable.