AirPort Extremes share FAT32 volumes via SMB (Windows-style file sharing). It does not share it over HTTP (the main web browsing protocol). So you connect to it via any SMB/CIFS client, not via a web browser.
So from Windows Explorer (NOT Internet Explorer), you could connect like this:
\\public.ip.address\share
Or from other tools, you could enter an smb://
or cifs://
URL something like this:
smb://public.ip.address/share cifs://public.ip.address/share
Note, however, that the ports used for SMB and its supporting protocols are some of the most-attacked ports on the Internet, so many ISPs block all external accesses to those ports automatically just to keep malware/botnet/script-kiddie/hacker traffic to a minimum. If you can't get this to work, you might want to check with your ISP to find out if they block those ports by default, and if they do, if they can unblock them for your IP address at your request. But you should keep a damn good password on that service.