I assume you're using the latest Dell driver for I8000 (A03) from support.dell.com. You have to turn on the SVideo out directly thru the ATi advanced properties. Dell didn't do any hotkey or other easy way to turn it on
Display properties -> Settings tab -> Advanced button -> ATi Displays tab and turn on the SVideo there.
If it still doesn't turn on, you may have a hardware problem and SVideo out may not be working at all.
Hope that works,