Duplex printing on a Samsung CLP-550n

I'm running a MacBook Pro with Leopard and have been printing (one-sided) quite happily to our networked Samsung CLP-550n. But, I can't print duplex, the option is greyed out in the printer dialogue box yet from other macs around the office its possible? The default on the printer is set to long-edge binding duplex printing.

Any help would be appreciated.