Pairing a Dualshock 4 (DS4) with the Playstation 3 (PS3)

Click on Accessory settings
Manage Bluetooth devices
Register a device

Note you must hold down the Share + PS3 button on the DS4 for about 3 seconds until the front display flashes rapidly.

Also note that the PS button and the rumble will not work.

If you need to reset the DS4, you can use a paperclip to press the reset button on the back near the R2 trigger.