How to enable external display on iMac with dead screen

On a iMac 24-Inch (Early 2008) Model:A1225 EMC:2211 I have a bad display - can just about make out the apple logo when I power up but can't see where to enter the password. I have a Mini-DVI to DVI adapter and connected another monitor that has been confirmed working on another comnputer. The signal seems to be recognised on the secondary monitor as the 'check signal cable' OSD goes immediately when the iMac is connected and reappears when the cable is disconnected. But the display stays dark. Perhaps I need to get into settings on the iMac to enable a secondary display. But how can I do that without seeing the screen ?