First of all this tool/script works only in firefox browser, hence you need to make sure that you have the latest version of your browser. Click on Help Check for Updates… in your browser to check out if the firefox browser is in need of a new update.
Next you need the Newer version of Gmail and you need to click on Never Version link in your Gmail account to make sure that all your gmail accounts are upgraded to newer versions.
Once the add-on is installed and your browser is restarted you have to open this link and click on ‘Install this Script’ link in the right sidebar to install the script. Make sure the tiny icon of GreaseMonkey is enabled in the status bar. Once done you will find that the ‘Log Out’ link will be replaced with ‘Change User’ link.