The Method getCurrentValue() is defined as an ordniary (non-static) method of the class. In order to execute it, you need an instance of HallLanceMemoryCalculator.
So try to instantiate HallLanceMemoryCalculator first:
HallLanceMemoryCalculator calc = new HallLanceMemoryCalculator();
double value = calc.getValue();
In order to make some sense, the example should have a constructor for storing the initial value. E.g.
public HallLanceMemoryCalculator(double initial) {
this.currentValue = initial;
}
In doing so, you can use the following main code:
HallLanceMemoryCalculator calc = new HallLanceMemoryCalculator(10);
int choice = displayMenu();
// some code to get the second operand (you don't need the string as param)
double operand = getOperand("");
// some switch statement to handle the choice
...
double result = calc.getCurrentValue();