To print a 3D object using MatterControl, you'll first need an object to print. Press the "Add" button at the bottom of the "Queue".
A file dialog will open allowing you to select a .STL, .AMF or .GCODE file. Once you've selected a file, it will be added to your Queue and available for printing.
To begin a print, check to make sure that the bed is clear, then press "Print". You can pause or cancel a print by selecting "Pause" or "Cancel" once the print has begun. After a print has finished, you can choose to reprint the object or select 'Done' to remove it from your print queue.