Noob question here. When you use a proton rocket and you spend the focus token to use, does that focus token can still be used to modify the attack dice used in said attack or is it not usable anymore?

You don't spend the focus to fire prokets it's just required that you have the token, so you can modify the attack.

This differs from proton torpedoes where you must spend the target lock to fire so can't modify the attack.

Yup. 'Attack: x' just means you must HAVE x to make the attack.

Only if the text of the card says to spend x do you have to spend it.

In general, do what the card says and only what it says. Don't assume anything about what it says, because, as in this case, your assumptions may be wrong.

Noob question here. When you use a proton rocket and you spend the focus token to use, does that focus token can still be used to modify the attack dice used in said attack or is it not usable anymore?

It's not spent to fire the Proton Rockets, so it can be used to modify the results. Compare it to Proton Torpedoes where the target lock is spent to fire the torps. And as Hobojebus pointed out, once you've spent a token to fire, it's no longer around to be used to modify the attack.

Thanks for the help