The Monitor is to be placed next to a Computer and it will display the text on the Computer on the actual Monitor block. You do not have to open a GUI to view it. You can place multiple Monitors adjacent to each other to extend onto a bigger screen.
Place at least one Monitor right next to the Computer. Then from the computer, enter this command:
monitor (side) (program to run)
(side) = top, bottom, left, right, front, back = whichever side the Monitor is at from the Computer
(program to run) = any existing program you want to display output on the monitor
Example: monitor top MyProgram