Don't do two getKeys. The program doesn't wait you know, so unless you hit the second key within about 1 millisecond of the first (and the debounce would stop that anyway) then you won't get into the second switch statement. You want something like:
void loop()
{
char key = keypad.getKey();
switch (key)
{
case '*':
Serial.println("[1] Addition");
Serial.println("[2] Subtraction");
Serial.println("[3] Exit");
Serial.println("Choose");
break;
case '1':
//Serial.println("ONE");
Addition();
break;
case '2':
Serial.println("Subtraction");
break;
} // end of switch
} // end of loop
Don't put a default in. The default case will be no keypress.
And you need to revamp how you to the addition test. For example, what if you randomly generate 8 and 6? The answer is 14, and you can't get that back with a single kepress.