This is not a macro that you run. It is an event procedure that will run, whenever your worksheet is changed. I am not sure if it is what you want. But to test it, follow the following procedure:
1. In excel hit, Alt+F11 to go into VBA
2. Look to the left in the project explorer and double click your worksheet where you want the fun to happen.
3. Look to the right for the big blank space and paste in the code.
4. Close VBA and try entering in numbers.