If you are referring to the 600EX (without the the RT), that model is optical controlled only like the 580 and 430 series and sold in markets (countrys) that do not allow Canon's radio controlled devices. I'm not an expert in radio controlled specifications so I don't know what specifically it is that prohibits it.
Here is a list of where they are allowed. I asked if the OP wanted RT because I read of people getting the non RT when they thought they getting the RT for a great deal. You have to watch carefully when you purchase on line like eBay.