You need an adapter because the PS3 doesn't have an IR port. You can get a usb adapter for cheap. Most people go with the Nyko Blu wave remote that comes with a USB adapter. It works pretty good but you cannot use it to turn the PS3 on and no PS button.

A better solution is an IR to Bluetooth converter. These will support all functions of the official remote. Unfortunately, they cost a little but more. There is a list and a full review here.