If you previously had this printer connected to the Mac via USB cable and then disconnected the USB cable and connected it to the Airport router, then you will have to add the printer again. Because the printer currently in Print & Scan on the Mac is for the USB connection and will not print to the networked printer.
So, open Print & Scan and select to add by clicking the plus button. With the Default view selected, the printer should appear in this view. If it does, select it. The Print Using menu should automatically select the appropriate printer driver and you can click Add to complete the print queue creation.
Now you should be ready to print.
The Airport Extreme uses the Bonjour protocol to advertise the printer connected to its USB port. If you cannot see this printer then you could have an issue with the Bonjour protocol. To check this, you can use Safair, but you need to enable the Bonjour bookmark via Preferences > Bookmarks, as shown below.
With this enabled you will now have a Bonjour entry and when selected, if the printer can be seen by the Mac it will be listed.
So can you see your printer using this method?