Click the links below for resolving backlit keyboard issues.
Is the keyboard itself not working, or is it the backlight that isn't working?
If it's the backlight, is the Dell ControlPoint software loaded?
If the original keyboard works when reinstalled, the new one is fault. If it doesn't, check the cable that connects the keyboard to the mainboard for damage - it is very easy to damage these (or the mainboard connector, which hopefully didn't happen) when you replace a keyboard.