It is important to understand that you can't just add a new key. You have to add all the keys in at the same time.
If you just add a new key, all of your existing keys will stop working.
So, before you start, get all of the keys for the car and ensure their batteries are ok.
Plug the MB Star multiplexer box into the OBD port of the car, turn on the laptop and boot up DAS.
Press unlock on the key, place it into the ignition and turn it to position 1 to power the car up.
When you get to the main screen, press F2.
Scroll down to "smart" and press F3.