I booked accidentally booked Penryn Exeter campasa instead of the main one Watch
The former case is a different matter entirely; you will have accepted a place on a different degree programme then. I would suggest contacting them as soon as possible - before attending the induction - to discuss your options and advise them you didn't mean to apply to the course on that site. They may be able to accept you onto the relevant course on the main campus, but there is no guarantee, although if the course was in clearing it's a reasonable chance. If they can't take you on the main campus course, then you can either go to the Cornwall campus course or withdraw from the uni and reapply for next year.
As above the campus isn't a gated community to actually stand on the land and e.g. enter the Forum etc. However, they aren't going to give you card access, set up your IT facilities etc etc if you aren't actually due to be registered on a course on that campus. In the same vein, you won't be entered to the exams etc if you aren't actually a student for that campus. Thus if you have applied for the wrong course then you need to negotiate this with them but they are under no obligation to accept you onto a different course (which it is, as the degree programmes between the campuses, even where they have the same name, are different as they have different options and naturally are taught by different faculty).