[globaleric]

Your hardware (graphic card) must support the extended desktop (dual screen) feature. I.e. PPC iMacs were only able to mirror the standard display on the external connector, unless some hack was applied.

If it's supported on your hardware and once you've connected and switched on your second monitor, select 'Arrangement' under the display preferences where you can define the extended desktop (2nd display) properties. Using the example in your pic, you'd want to drag the extended desktop to the left side of your standard desktop with the menu bar.
After saving/closing that, start PT and drag the mix window and maybe some plugins to the left - they'll appear on the 2nd display. Those new window positions are stored when the session is saved.

From now on it's a good idea to have the 2nd monitor already connected and switched on before launching PT.