Set your browser to http://sourceforge.net/projects/mmmysql/ and download the latest version of the MM.MySQL driver to a convenient location (C:\TEMP\mm for Windows, or /tmp for Linux).
Open a command prompt (Windows) or use the command cd /tmp (Linux). Run the following command:
C:\TEMP\mm>jar xf mm.mysql-2.0.8-you-must-unjar-me.jar
This creates a directory called mm.mysql-2.0.8. You'll find a file called mm.mysql-2.0.8-bin.jar inside it. Copy that file to the lib directory underneath your Tomcat install. You've just installed the MySQL driver.