So you have an Airport Extreme correct? Your private IP address is find-able in System Profiler. To get there, click the Apple icon on the top of your screen, then select About This Mac. It will appear with a small window, click more info on the bottom. On the left side of the new window that appears look for a dropdown section call Network. Click on that Network dropdown. Nothing under it. Then look for the IP that is located next to what it says is IPv4. That is your private IP.
for those trying to figure out their private ip in Windows, go to the Start Menu, click Run, then type in "cmd" without quotation marks and press enter.
a command window should pop up and then type "ipconfig" without quotation marks and press enter. find your IPv4 in this format:
IPv4 Address . . . . . . . . . . . . . : x.x.x.x
that's your private ip.