I'm very surprised there is not one straight forward answer to this online.
Navigate to your Airport Utility
Click Manual setup
Navigate to your "Disks" icon at the top
Click the "File Sharing" tab
Check the box next to "Share disks over ethernet WAN port"
Click update at the bottom.
--This enabled sharing of your Airport disk over the internet.
After airport refreshes, it will revert back to the "Airport" icon.
At the bottom of the window, there is an IP address, xxx.xx.xxx.xx <- copy it
After you have your airport IP copied, click on finder.
At the top of the screen, click Go->Connect to Server (or just cmd+K)
Type in - afp://xxx.xx.xxx.xx (your copied IP address) and then click connect
It should prompt you for a user name and password if you have it set up.
After logging in, it should mount your drive.
Done!
Easy.