Where is the anode rod if it is not visible it should be on top

The anti corrosion anode is inside the tank, access from the top of the tank.

https://tse1.mm.bing.net/th?id=OIP.M...0&pid=3.1&rm=2

Look on the top of the tank, there should be a large nut, 1 1/8". Lots of torque needed to get the nut loose