Q: Use 21.5 in. iMac as an external display for macbook pro

Hi,

I want to use my 21.5-inch iMac (2010) as an external monitor/display for my 13-inch Macbook Pro (mid-2011). After reading a couple forums, I thought the answer was to buy a Mini displayport-to-Mini displayport cable and connect one end to the Thunderbolt port on my macbook and the other to the mini displayport on the back of the imac. But it didn't work. Nothing happened after restarting the imac and the macbook. I looked online and read that I needed to press command + F2 and that would activate Target Display Mode but nothing happened. So I put the iMac into TDM through System Preferences. When the iMac restarted, there was a gray screen with a large white version of the FireWire 800 port icon .

So my question is:

(1) is this even possible?

(2) if so, what cord do I need to get?

Thank you in advance for any help!