Even though you are using an adapter, the signal being put out is still meant for a DVI screen. There usually will be a display setting that will tell your display adapter to put out either a s-video or composite video signal from the DVI output.
On the apple adapter website, it says the only systems compatible are the Mac Pro (with ATI X1900 XT), MacBook Pro, Mac mini or Power Mac G5 with DVI port..
Check the specs to see if the port is DVI-I or DVI-D. If you have a DVI-I then you should be able to change a setting somewhere to change the signal from digital to analogue. If it is a DVI-D port, the adapter will not work, as the video card will only output a digital signal.How do I get my Sony HDTV to serve as an external display for my iMac?
When it's hooked up, you should be able to edit the display mode in the system preferences. Click on the apple icon and then click on System Preferences. You should see a window with a bunch of icons come up. Look for the Hardware sub title and click on the Displays icon. You should be able to edit your displays in there choosing either extended desktop or clone.