IN the magazine or ON should be used?

You would use "in" when referring to something being inside a magazine.

Examples:

* "I saw an article about space travel in the magazine."

* "There was a coupon for a free pizza in the magazine."

"On" is used when something is placed on top of the magazine or if you're talking about the magazine being on a specific surface.

Examples:

* "I left the magazine on the coffee table."

* "The advertisement was printed on the back cover of the magazine."