Install: Download and unzip it to your wordpress' /wp-content/plugins/ directory.
Activate it in Plugins menu. View its options in Options > LaTeX tab --
It is actually used as a mimeTeX plugin rather than just a plugin. My contribution is technically trivial. But, you must need it if you change from a light theme to a cool black one while find the default mimeTeX images are black in front and transparent in background. This plugin provides mimeTeX's \reverse and \opaque options to tune your math forms to your wp theme without editing them one by one.
The 2nd function is the option for your cgi URL. The default is http://www.forkosh.dreamhost.com/mimetex.cgi You can DIY one following Forkosh's instructions and then set it like /cgi-bin/mimetex.cgi It can help your visitors not to cost Forkosh's unselfish bandwidth.