Use iMac monitor as security monitor

I am trying to connect my 2011 IMAC to my ADT Security DVR to use for monitoring security cameras. I used the mini display port vga connector to connect to the external DVR. However I am not getting any video from the DVR. They IMAC only starts switching around different display resolutions.

Any suggestions on how to accomplish this would be much appreciated

What iMac and what connection are you using?

Imac 2011 27 inch, Intel Core i3

OS X 10.8.5 (12F45)

Connection: mini-display port from iMac to VGA port on Security DVR

You are trying to go "from" the VGA port "to" the mdp on the iMac.

You will need to find a converter to convert VGA to a displayport signal.
I'm not aware of anyone making such a converter.

The mdp to VGA adapters you can find everywhere will not work - they are one way - displayport "to" VGA adapters.

You might start here - link.

The easiest and cheapest way is going to be buying an el cheapo VGA compatible monitor.

edit:
(Also, the only Core i3 in a 27" would be the mid 2010 model - sold thru beginning of May '11.)

Thanks, it looks like the monitor is the way to go.
Correct on the Core i3.