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.