Find the MAC address of the camera.
In Airport Utility, edit the base station. On the Network tab, set up a DHCP Reservation for the camera using the MAC address. It's not really a static IP, but will always give the camera the same IP address.
If you really need it static, give it an address with the last octet between 201 and 254. If you have changed the default addressing of the AE, you'll have to use an address above the last address you used in the Network Options:
In that example posted above, set the camera to 192.168.1.201 or higher (up to 254).