Connecting an Macbook Pro to an older iMac

Hi,
I've got a Macbook Pro 15" (early 2011) and I want to connect it to an older iMac at school so I've got a bigger screen for photoshop.
But the problem is that my MBP has a Thunderbolt port, but the iMac doesn't.
The iMac does have a Mini Displayport

At my local apple store they sold me an Mini Display -> Minidisplay cable and said it should work from the moment I connect it.

I've tried it, doesn't work.

Trough google I've found that you've got to press CMD + F2, But that doesn't work either.

Is there a way to do this? Or did the store just ripped me of?