In you haven't already, check out this Port Forward reference:
Port Forwarding for the Apple AirPort Extreme Base Station
As you have already noticed, these were instructions for the older 802.11g AirPort Extreme Base Station (AEBS).
Try these for the 802.11n version ...
AEBSn - Port Mapping Setup
To setup port mapping on an 802.11n AirPort Extreme Base Station (AEBSn), either connect to the AEBSn's wireless network or temporarily connect directly, using an Ethernet cable, to one of the LAN port of the AEBSn, and then use the AirPort Utility, in Manual Setup, to make these settings:
1. Reserve a DHCP-provided IP address for the Slingbox.
Internet > DHCP tab
o On the DHCP tab, click the "+" (Add) button to enter DHCP Reservations.
o Description: <enter the desired description of the host device>
o Reserve address by: MAC Address
o Click Continue.
o MAC Address: <enter the MAC hardware address of the Slingbox>
o IPv4 Address: <enter the desired IP address>
o Click Done.
2. Setup Port Mapping on the AEBSn.
Advanced > Port Mapping tab
o Click the "+" (Add) button
o Service: <choose the appropriate service from the Service pop-up menu>
o Public UDP Port(s): 5001
o Public TCP Port(s): 5001
o Private IP Address: <enter the IP address of the Slingbox>
o Private UDP Port(s): 5001
o Private TCP Port(s): 5001
o Click "Continue"