If you" tried doing the things they say to do" maybe you should move on to these Apple troubleshooting articles and see if there is anything in these articles that "they didn't say to do".
iOS Device not Recognized in Mac OS X
iOS Device not recognized after restart in Mac OS X 10.6.8
iTunes: How to remove and reinstall Apple Mobile Device Service in Mac OS X 10.6.8 or earlier
I assume that tried a different USB port on your Mac and that you have tried using another cable as well. But if you haven't tried those things - give tham a shot. If you have a cable from an iPhone or an iPod Touch - you can try one of those cables - they will work.