Note: Make sure you already have Bukkit enabled on your server.
1. Find the plugin you like on the Bukkit Plugin List page.
2. Download the plugin to your computer. If it's a zip file, extract the contents so you can access the jar file.
3. Login to your control panel
4. Click files
5. Click FTP files access
6. This opens the upload area where you will see your server to connect to, Host, Port and FTP Username (the username is different from your control panel login name)
7. Follow this link for detailed information on how to use Filezilla.