I can't see where lngTotal is getting assigned originally, so I'm not sure how the branching logic will be handled.
But generally, I think the problem is that you can't actually execute the "add" process when you press the "+" key. That isn't how it works on most calculators anyway. You should handle a sequence of key presses like this:
You have to place the first value into a variable and capture the operation (+,-,*,/,etc.) to another variable. Then you can perform the desired operation (whether or not the user enters a different value).