Lift up at the rear of the case and work your fingers along the sides, freeing the case as you go. Once you have freed the sides, you may need to rock the case up and down to free the front of the upper case.
There are four plastic clips above the DVD slot, and another above and to the left of the IR sensor. These clips can be very difficult to disengage without prying. They can also be difficult to re-engage during reassembly.