If you have the display attached, and the display power is on - can you see the display as a device in your System Information, then Graphics/Displays tab?
Are you sure that the display is working? Do you have another computer that you can try the display, or even a friend with a computer that can help you?
Have you tried shutting the iMac off, then attaching the Hanns with adapter, make sure that the display is on, then boot your iMac ?
If the display works when connected to another computer, then, yes, you need to get a different mini-DVI to VGA adapter.