how do i add an external hard drive to my network

If your external HDD has a USB interface and is formatted in either HFS+ or FAT32, you can connect it directly to the USB port on your 802.11n AirPort Extreme Base Station (AEBSn). You would then use the AirPort Utility to configure the AEBSn to share out the HDD over the network.

Please see the following Apple Support article on how to mount the HDD on your computer.