To add cards to a deck, go to the Build page for the deck. If you have created a new deck you will be taken to the deck's Build page automatically. For an existing deck, go to that deck's Details page and then click the 'Build' link on the right of the deck's title.

Type the name of the card you want to add into the search bar and any cards that match will appear below. If any of those cards are in your collection, a purple box will display how many copies you have.

Click the card you want to add. 

Use the drop down menu to the right to determine whether the card is added to the main board, side board or a custom board.

Click the 'Add One' button and a single copy will be added. To add four copies, hold CTRL or CMD and the button will change to 'Add Four'.