Is the 6600 one of the models that do not have WiFi? If so, you must connect it to your router via Ethernet. If it has its own Wifi, it needs to be connected to the same WiFi system as your iPad.
Once the HP 6600 has a connection to the internet, you can access the setup or preferences menus on its control panel, and look for an option to update it. Updating will most likely fix the problem.
One other possibility is that your Wifi Router does not correctly handle Bonjour data required by AirPrint to locate the printer. If that is the case, contact the router's manufacturer to see if there is a change to its setup or a firmware update that will solve the problem.
Is the HP on the airprint list?
iOS AirPrint Printers http://support.apple.com/kb/HT4356
How to Print from Your iPad: Summary of Printer and Printing Options
Print from iPad / iPhone without AirPrint
How to Enable AirPrint on a Mac and Use Any Printer