How insert “+” from MacBook italian keyboard?
If you actually can't find the key "+" by System Preferences > Keyboard > [X] Show keyboard in menu and testing all modifier keys with Apple's keyboard overview app, you might use Terminal.

Open and type xmodmap -pk.
This will give you a list of key names &amp; values. Copy contents to BBEdit/TextEdit, search for "plus".
If it's in a first place the key must be accessible directly.
Second place will be combined with shift, third place with option, fourth with shift &amp; option.

So: look for the line containing "plus", somewhere, and note down the first key's name.
That's your key "+" – to be pressed with or without modifier(s).