Use this code (you can type it, or click the "Add a card" too above the list of emoticons):
[card]***[/card]
– where *** is the name of the card you want. Make sure you spell it right, and always preview before posting!
You can get upgraded cards by adding ^ after the name, unless it has a unique name.
E.G.:
[card]Nightmare[/card][card]Nightmare^[/card]