using Imac 21'5inch as a second display.

Hi Guys
I have a Mac pro and I want to use a 21'5inch Imac as a second display for editing. How can I do that? I connected both with a minidisplayport to minidisplay port cable, but is not working. Here is what I have:

a Mac pro:
Mac OS X version 10.6.8
2.8GHz Quad-Core Intel Xeon.

a Imac 21'5 inch
Mac OS X version 10.6.8
2.7 GHz intel core i5

The Imac has aThunderbolt port, but the Macpro doesnt. Instead the Macpro has 2 minidisplay ports.

Any ideas?

Thank you

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