Right click on the desktop, properties, "extend my desktop to this display" on the 2nd display.
You have to be specific if you want help. Does your Macbook pro have Nvidia or ATI graphics? They have different control panels; and you may have to use them depending on the Plasma's resolution.
On XP (Service Pack 2), open your Hardware Wizard (Control Panel to View My System Info to Devices to Show Hardware). Click that and it should say in the list something like "LCD" or "Display." Click that and the Plasma should be there. Click on Properties and there is your answer.
its not working, or showing up any more. the right click to display worked, but now my xp crashed and it wont.
i go display selector, and select tv + laptop, but it wont detect my plasma. wtfÉ!