Jive
To run the Plugin Manager, use the directory from your mt.cgi installation with mt-pm.cgi at the end. So if you type in http://mt-plugins.org/mt/mt.cgi to get to your MT installation, you’ll use http://mt-plugins.org/mt/mt-pm.cgi to run the Plugin Manager. The login is connected to your Movable Type (mt.cgi) login… if you aren’t “remembered” from the mt.cgi screen, you’ll be prompted to log in.
Once you are logged in, you’ll have a list of installed/uninstalled plugins. The Manual Install option will let you connect your currently installed plugins with the directory. By selecting a plugin to match each file in the directory, and telling it which version you have installed, this will allow the PM to recognize its details and add it to the list of installed plugins.
You can also click on any of the plugins on the first page to get more details and do installation right from inside the PM.