Just opened this expansion and i was wondered that there weren't any rulebook inside the box. Is it missing in mine or isn't there any?
There just isn't any. Some guidelines about mixing these in with the main components would have been nice, but I think the idea is just to leave it up to the players. I think once you've played the base set a few times and are comfortable with it, just mix in all the cards from the Alien Pack, or just mix and match to get the kind of game you're looking for.
No gameplay rules are necessary - it's just more stuff that plays by the same rules as you get in the base set.