Magic 2013: What do I do with card .xml files?

I've just recently started playing DotP 2013, I know it may be a little older, but I bought it due to a Steam sale. But that's off subject. I wanted to know how I put in custom cards like one can put in the DLC decks made by the modders here. I've been using one of the deck creaters on this site and I assume that it uses the cards from all the decks in the game, including the one for modder's DLC decks. But I wanted to know if it is even possible to put individual cards into the game or be usable from the deck builder I got. The card generators on this site all give me the card in a .xml file so I was wondering what I did with those. The deck builder program I've been using is Riiaks DotP 2013 Deck Builder v1.4.2.1 x86, if that will help with anything. Thanks for any help!