You have a very common problem. The problem is either
a.) You have a defective charger, I have see this many times before. If this is the case, you will most certainly need a new charger. To see if you do, try and borrow a friend's or neighbor's charger. If another charger works then you will need to look into getting a new one.
or
b.) Your dc-in board on you computer is defective. This part requires a removal of the bottom cover and logic board. The part can be found here:
[product|IF163-011]
and the guide here:
[guide|7660]
This isn't the easiest repair, but, if you take your time and pay attention then everything will go smoothly. Only proceeded with "b" once you know it is not your charger.
Hope this helps