If you want to use the bridge to accept the wireless LAN client association, YES you do need ACU (Aironet Client Utility) to be installed at client side. In ACU you can configure WEP keys.
Pl. visit following url for more on that
http://www.cisco.com/warp/public/102/wlan/confwep.html
You will find useful tips on that at following location
http://www.cisco.com/warp/public/102/