Its the language thats making it confused.
Try some of the following keys:
The key next to Z (on the left of Z), on a US keyboard its marked |\, try pressing CTRL and SHIFT when pressing the keys.
The key next to the right shift marked ?/
the keys next to enter on the left marked @' and #~
One of these will be mapped to the tilde key in Italian, sorry I can't be of more help, but I've never used Italian.
Best thing to do: Open notepad and press the keys, using shift and control as well until it puts a tilde on the page, then remember what you pressed for FO3.
what you means "Open notepad and press the keys, using shift and control as well until it puts a tilde on the page, then remember what you pressed for FO3."