I'm not sure this is the correct part of the forum to post, as its a rather generic question about the HT12D - perhapsa the 'projects' part of the board would be more appropriate?. But I believe you could do what you want by using a full bridge (H-bridge) transistor driver (depending on the power of your motor).
The HT12D has 4 digital CMOS outputs, which are active low. So you would need a relatively simple H-bridge which accepts two inputs, one for 'forward' and one for 'backward'. Depending on the transistors you use, you may need to invert these signals (as they are active low).