Your iMac doesn't have an internet connection? I think it would be best if you could get a direct connection for the iMac, because while 3G speed is good enough for mobile computing, it's much slower than what you would want for desktop computing.
If you do have to tether, then the only way I know is to jailbreak your iPad and install an app called MyWi. This turns your iPad into a WiFi hotspot, to which your iMac should be able to connect. However, you should keep in mind that your iPad's battery life is likely to take a big hit.