Where is the hash key on MBP US keyboard? (Alt 3 doesn't work)

Hi,

I've just bought a 2 yo Macbook Pro from an english friend of mine, so I guess I have US keyboard (I'm italian), and I just can't figure out how to make the hash key show up! I've read around the internet that the combo is Alt+3, but that's what I get when I do that: £.

Anybody has a clue?