Go Here
http://intensedebate.com/install

Upload to wp-content/plugins

Activate in Plugs tab

Go here to Get your KEY
http://intensedebate.com/userkey

This is your password. They forgot to mention that. It won’t work if you try to login with your username and password for IntenseDebate. You need that key

Clearing your WP-Cache can help with getting the plugin to show up after you have installed.