You don't necessarily need an FTP for a small plugin like this. In the download package (after you unzip it) you'll notice a folder called 'Upload'. Open that folder and compress the contents into a .zip file - there should be 2 folders in there called 'admin' and 'inc'.
Upload that .zip file to your forums root directory (
http://www.website.com/forum/). Your root directory is the location where your MyBB installation is installed.
Unzip the .zip file.
Go back to your forums and navigate to Admin CP > Configuration > Plugins
You should see the plugin there. Press 'Activate & Install'. Once activated, navigate to Admin CP > Configuration > Settings and down the very bottom of that page you'll see the settings for the plugin.