Method to add multiple cards

You mean adding multiples of one card?

If you want to add 24 mountains for example, you have to just add one from the Add Interface widget. Then you will notice in the listing, to the left of the card name 3 counts - on is the number of that card that you have in the inventory, the second one your wishlist, and the last one in bold is the number of that card that are in your deck.

All these counts are editable directly by just clicking on them, writing a new number and pressing enter.

If you meant adding multiple cards from your inventory to a deck, then - while looking at your inventory, select the cards you want to add, then use the Card Actions dropdown, and select Move to Deck.