If this is an older printer, you might need a Printer Driver for you iMac. Here is a link to a driver for this printer that works on OS 10.6. I didn't see one for OS 10.7, but this driver might work on it.
If the link doesn't work try this link:
Type your Printer model number and click search. That should pull the list of drivers for that printer. Click on the Mac OS 10.6 driver.
Welcome to the Apple Support Community, Scott!
Congratulations on your new iMac. From your profile, it looks like you may be new to Mac and it's OS. So, take a look at Apple's KB article Mac 101: Printing (OS X Lion) and in particlular the info on How to add a printer. Note: Apple released update printer software for HP printers late last week, so be sure to run Software Update as the article suggests.