Can't connect iMac through Pronterface

Hi, I am a total newbie to 3d printing. I just received my Plastic Scribbler Asterid 2200 printer and am trying to connect it to my iMac through Pronterface (MacPronterface). The iMac is running OS X (10.7.5 - Lion). I plugged the usb from the printer into the iMac and started Pronterface. At the top left corner of Pronterface is the "Ports" button and a drop down menu. When I click on the drop down menu, it says "None" for Ports. How do I get the com ports to show up so I can select one? As you can probably tell from my question, I am not very tech savvy. Oh, this may help, the printer, usb cable and Pronterface all work perfectly on my wife's PC which is running Windows Vista. Any help will be greatly appreciated. Thank you.

Jim

A quick update…I downloaded the CH340 driver for the arduino board and now I see a port listed in the drop down menu called "dev/tty.wchusbserialfa120". If I select this port and click "Connect", I get the following error messages in Pronterface:

Error - Could not connect to dev/tty.wchusbserialfa120 at baud rate 250000.

Serial Error - Could not open port dev/tty.wchusbserialfa120. No such file or directory.

Thanks,
Jim