Hi Lara -
If you are attempting to change settings on the Airport Extreme remotely, all you really need to do is to set the WAN access. Then use the Airport utility and enter the Dynamic DNS - or public IP to access the Extreme - FROM A REMOTE location.
Back to My Mac - essentially allows you to access files/documents on your home computer via the Internet. To accomplish this, set the BTMM ON in you iCloud Preference, and in the Airport Exteme - log into your iCloud account. In System Preferences, set Sharing - turn on File Sharing, and share a folder. Now when you are away from your home computer, you should see your computer showing up in the Finder sidebar.
If you wish to access another device on your local network using a web browser, then you will use the Port Forwarding settings. If for example, you wish to log into a Web page on a web server in you local network, then you would port forward port 80 to the IP address of the server on your local network. Port 80 is used for web access. Ports 20-21 is used for FTP (File Transfer Protocol - but is broken the Airport Extreme 6th Generation), Port 23 is used for Telenet, etc.
Hope this helps.