Ummm... think twice about removing the outer cover of your oven. Microwave ovens, by the nature of their design, can give you a serious, even deadly electrical shock, even though you may have take the precaution of unplugging the device.
Additionally, the fuses used in microwave ovens are unlike most other fuses. It will be ceramic or glass with wires at the end. It may not (especially if ceramic) indicate that it's faulty by appearance. Usually, if the fuse has blown it will cause scorch marks on nearby surfaces.
If you do decide to progress, the fuse location will be just a few inches inside the outer cover along the power cord. Be especially cautious of the box called the high voltage capacitor near the Magnetron, which retains high voltage. It can be safely discharged, but I'm not about to describe that process for you...(Wouldn't you like one of those new, shiny, mulitfunction ovens any way?)