This is one of those things where you have to tell us what country you're from, dear...
In the US the hash (#) is shift-3, just like on any other US keyboard. But if you're in the US, I'm assuming you can see that the hash symbol is printed on the 3 key, and you wouldn't be asking this. So where are you from?
Or, if you don't want to tell us where you're from, open sys prefs -> international -> input (last tab over I think). Then turn on the keyboard viewer in the list, and select it from the flag menu in the menu bar. Then you can see what keys make what symbols and you can find your hash mark.