As Corey Says but first off do the following:
On your windows desktop right click for the drop down menu and select "Display Properties"
when this loads up you will note (with the screen connected) that you currently have your "Multiple Displays" option set to "Duplicate these Displays" change this to "Extend These Displays" now click apply, note the resolution (eg 1024x768) of the extended display before closing the Display Properties Window.
Now goto TD and do as Corey states, entering your noted Windows extended resolution so it matches your TD extended resolution.
Every time you start TD you will need to move your game screen onto the extended display, I find it best to have the control buttons on the dashboard so you don't need to go back and forth into screen properties each time.