I have a new iMac running OS X 10.8.3. I am adding a Windows network printer by IP address. The iMac sees the printer, but when I print it says "Printing-The printer is in use." and hangs forever.

Hello there SchoolAdmin52,

Thank you for using Apple Support Communities 🙂

I found this article for you to help troubleshoot the issue. It is named Troubleshooting printer issues in OS X and you can find it here: http://support.apple.com/kb/TS3147

Follow these steps until the issue is addressed:

1. Make sure that the printer is powered on, has ink / toner, and that there are no alerts on the printer’s control panel. Note: If you cannot clear an alert on the printer's control panel, stop here and check the printer's documentation or contact the manufacturer for support.
2. Ensure the printer is properly connected to a USB port on the Mac or AirPort base station / Time Capsule. If the printer is a network-capable printer, make sure that it is properly connected to your home network.
3. Use Software Update to find and install the latest available updates. If an update is installed, see if the issue persists.
4. Open the Print & Scan pane or Print & Fax (Snow Leopard) pane in System Preferences.
5. Delete the affected printer, then add the printer again.

If the issue persists, try these additional steps:

1. Reset the printing system, then add the printer again.
2. If the issue still persists, reset the printing system again. Download and install your printer's drivers. Then, add the printer again.
3. Contact the printer vendor or visit their website for further assistance.

Regards,

Sterling