Back to My Mac may not work as expected with AirPort Extreme 802.11n (2nd Generation)
AirPort Extr Base Station, Back to My Mac
If the computer you are attempting to connect to via Back to My Mac is connected to the Internet through an AirPort Extreme 802.11n (2nd Generation), the connection may not be successful. This can occur even if you have enabled NAT-PMP.
Install the Time Machine and AirPort Update (via Software Update), and also update the firmware on your base station to 7.3.1 (by opening AirPort Utility).