I have just bought a Macbook pro and I have to program. But I can not figure out how to insert this symbol "+". On the net I could not find anything.
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
This will give you a list of key names & 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 & 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).
If for some reason, you are pressing the key which has + printed on it, but you are getting something else, go to system preferences/keyboard/input sources and make sure that there is only one item on the list, and that it is called "Italian" (and not "Italian Typewriter").