"In the campus" is not correct.
Say "on campus" or "on the campus." Both are correct.

There is no rule, but these guides will help you:

Usually use in with something that you can get inside of.
in the library, in the kitchen, in the dorms, in the cafeteria, in the pool, in the elevator, in the department store. 

Usually use on with open spaces
on the campus, on the grounds, on the playground, on the soccer field, 

Sometimes the article, the, is omitted through common use. People just say it that way and the shorter form becomes accepted. There is no rule for this. 
Examples: on campus /on the campus (both correct) 
Say on public property, on company property, on private property. 

Here are some common exceptions:
in the park, in the woods 
on the stairway, on a ship, 

Here's a really silly exception: 
You travel on a plane or by plane. 
Of course, you don't sit on the wings, but that is how it is said.

Hope you are now are certain, without a doubt, that on campus / on the campus are correct.