Yes, "in" would be used with the print media: "in the newspaper," "in the New York Times," "in a magazine," "in the National Geographic," "in a book," "in the dictionary," even "in my electronic dictionary."
In the title of your posting, you want to know if the expression is "in the plan" or "on the plan." This depends on the type of plan. You would say "in the plan," or "in someone's plan," if you are referring to a plan for action:
I wasn't in the company's plan for reorganization, so I was laid off.
We're figuring out where to go on our road trip. We were planning to spend some time in Florida, but we've shortened our trip. Florida isn't in our plan(s) any more.
Although the team won, it was by accident. They had planned a different strategy. The fact that the runner scored in the surprising way he did had not been in the plan.
On the other hand, you could use "on the plan" when you refer to someone's being listed in a kind of set program, such as insurance benefits:
I can't go to that doctor. She's not on my plan.
or, on other kinds of set programs as referenced here in examples from the Collins COBUILD online*:
...identification only edged red on the plan annexed hereto hereinafter called...`
...he repeated finding them on the plan he had roughed out over the years...
Sometimes "in" or "on" is connected to the verb or adjective it follows, not to the word "plan." Maybe it is in cases like this, as in examples from the Collins COBUILD*, that makes one uncertain:
...more than had been envisaged in the plan...
...PAMF physician participates in the plan...
...exceed 19.9 or 22.9 depending on the plan for which you are approved...
...why the Italians are so keen on the plan In Britain, two in every nine homes
...across the border depended on the plan that he and Chavez had discussed...
...for the USSR, based largely on the plan which had already been put forward...
As you can see, there's more to be considered that only the preposition "in" or "on" in front of the words "the plan." It's necessary to have more context to know which preposition to use.