To create a new blank row to the Rundown, press the plus button in the toolbar at the top.

Add a new row linked to a Card, by pressing the search-plus button and search for the Cards you want to add. 

Click the Cards in the search window to select them, any you have selected will display "Added" next to the Card.

You can also add a new row directly to a specific segment by right-clicking on the segment and choosing "Add Row" or "Add Cards".