You can use the built in installer and upgrader, or you can install the plugin manually.
- You can either use the automatic plugin installer or your FTP program to upload it to your wp-content/plugins directory the top-level folder. Don't just upload all the php files and put them in
- Activate the plugin through the 'Plugins' menu in WordPress wp-admin
- Visit your GC Message Box options (Plugins - GC Message Box) - required for plugin init
- Configure any options as desired, and then enable the plugin