Hi! I bought a VESA adapter kit but apparently the wrong one, as it appears to have been designed for the 20" or 23"/30" HD displays. After a few hours with Apple Care and others, I found this:
http://manuals.info.apple.com/en/iMac24inch_VesaMountAdapterInstallGuide.pdf
There is a drawing of the card you need to disengage the stand and the TORX screw removal tool you will also need.
Please note that I have a 24" LED Cinema Display, not an iMac, and I'm assuming the base stands are the same--in pictures, they look the same to me.
So, I'm not sure how you can get the card (though the manual mentions getting a replacement), but I thought this might start to help.