I own a 27" iMac, 2010 model and a 2011 15" MBP. My Belkin Mini DisplayPort to Mini DisplayPort cable just arrived this morning. I have plugged the two computers together. I am unable to use the Target Display Mode on my iMac to enable fullscreen viewing of my laptops desktop on my iMac. Right now it's just a dual setup between the laptop and its, "freespace" on the iMac. How can I shuffle between my iMacs desktop and then my laptop on my iMac screen?