Bruce is correct, but be certain you select IP > HP Jetdirect-Socket. Then enter the IP address with the port number separated by a colon " : ".
You could also use the CUPS web page to add the printer. At the Device menu, select 'Appsocket/HP Jetdirect' and then at the Device URI menu, after socket you can enter the IP address with port 9101, such as
socket://ip_address:9101/
Do you know if the printer is using a Fiery controller? I know the Fiery's offer three ports alternate to 9100 so that you can connect to a specific queue. 9100 will print to whatever has been selected as the default queue in the controller config, such as print, while 9101, 9102 and 9103 are the direct, print and hold queues respectively.
Pahu