Well, it should be in the manual, which isn't posted on the MTH site. There's a jumper on the tender board that has to be removed for DCC. Here's the location of the jumper on the tender PS/3 board. On the one I have in my hand, it's actually silkscreened on the PCB "OPEN FOR DCC".
The picture appears to be an earlier one as it doesn't have that legend.