It only shows up when you pass a parking spot and the sensors/computer recognize that it can park there.
- Drive past the potential parking spot at <10mph
- If the sensors recognize the spot, a [P] will show up in the dash
- Stop
- Shift into reverse
- "Autopark Ready" will appear on the center screen with the parking spot
- Press "Start"
- Wait and watch
That said… if you have an AP2-equipped car, then Autopark hasn't yet been enabled.