Problem connecting over the internet to my AirDisk via AEBS

Hi all. I followed the guide over in the MBA Forum to forward the right ports in order to access my USB hard disk over the internet. I can access it on LAN just fine, and it all goes swimmingly until I'm asked to enter the "Private IP Address" of the device. The guide states to enter an IP address in the fashion of "192.168....", but as you might be able to tell in the picture, the first three octets of my address field are greyed out and unchangeable, and only let me enter an address in the form of "10.0....". Following the guide to the end is futile, and device connection doesn't work as advertised. Am I missing something? Is my problem clearly with entering the wrong IP, or what? Thanks in advance!

 

Perhaps it's not called AirDisk, but it is possible to access a networked drive via IP according to Section 5 of this guide. I feel like I've followed the instructions to the letter, but I seem to be facing inconsistencies. Maybe it's a newer firmware that doesn't allow this functionality, but I doubt it