If you're using the OBDeleven NextGen device, you don't need to pair it via Bluetooth. You just need to turn Bluetooth on, click "Connect" on the OBDeleven app, and choose your device from the list.
However, you do need to pair your device via Bluetooth if you're using the OBDeleven FirstGen (out of stock already). The code for pairing will be either "1234" or "0000".