I'm having a problem with my unlocked/jailbroken 3g iphone to be recognized by my imac?

[dpcalif]

I've had my iphone for about a year now (I bought it through ebay) and now when I plug it directly to my computer (imac) it doesn't do anything at all. I tried restarting both devices, but didn't help. I remember the seller stating that I cannot update my itunes, or it will then lock itself.....any suggestions on what to do here??? Thanks so much