If it's connected via USB, you'll need to assign a static IP to the host computer. USB devices connected to computers cannot be assigned IP addresses themselves.
You can use DHCP Reservation in the AirPort Utility and assign your computer a static IP based on its MAC address.
Edit: I'd previously assumed you had an Apple AirPort Base Station. Other routers should have something similar to DHCP Reservation (you'll need to check your router's user guide). The only way to assign an IP address to the printer directly is to have the printer itself connected to your network either wired or wirelessly.