Looking at the specs, the MS-6421 is a barebones HTPC chassis (and a nice looking one at that). If you are trying to use a remote with it ) either a supplied remote or \Microsfts own Media Centre version, you should be aware that it will only work with Windows Media Centre (unless you have a specialised remote). If you run Windows XP Media Center Edition, Vista Home Premium and W7 Home Premium editions that you should be able to use your remote when Windows Media Center is running.
When you say that you see a light, do you get a flickering light from your computer when you pint the remote at it and press a button? If so then it is workign correctly (the signal is being sent from the remote and the computer (at least the chassis sensor) is receiving it.