Here is the instruction on programming 3 button remote key for Mercedes Smart Fortwo 451 up to 2013 by using original SuperOBD SKP900 auto key programmer.
Instruction:
Note: Do follow every IGN ON & OFF operation.
Connect SKP900 with vehicle via OBD diagnostic socket
Select IMMOBILIZER
Select Vehicle Brand: SMART
Select model: BR451
Loading
INFO: Remote will adapt when key programming finish
Select Immobilizer system
Choose OBDII adapter, Turn ignition ON
SKP-900 will display VIN/FIN code
Select PROGRAM KEY
Adapt 6 keys most
Turn ignition ON Programming… Turn ignition OFF Turn ignition ON
Programming…
Program Finish. Press YES to program next key (or NO to go back).
Turn ignition OFF
Insert new key. Turn ignition ON
Programming…
Turn ignition OFF Turn ignition ON Program Finish.
Test the new programmed remote key.
PS:
SKP900 key programmer support following Smart cars list.