I believe all you have to do is open the Airport Admin Utility, Show All Settings, go the to the Airport tab, and check the box "enable encryption (WEP)". As I recall, you are then prompted for a password.
Once the ABS updates and restarts, your Mac should detect the network is encrypted and request a password (giving you the option to store it in the keychain.)
(That's based on a Snow (aka Dual Ethernet) ABS with v3.1 Airport Admin Utility under OS X 10.2.6.)