A little heads up : the simulation will surely be slow!
You can write a 4-bit number at the bottom left and visualize it at the bottom right before loading it in the memory via the push-button at the right.
The sum of the two numbers in the memory will appear at the top right.
You can see in detail how to make a bitwise addition here : http://everycircuit.com/circuit/5253252183228416
