How do you get into the slateport museum?

You can't actually "get into" the Slateport Museum in Pokémon Ruby, Sapphire, or Emerald. It's just a building with no entrance or interaction.

There is a Pokémon Contest held at the museum, though. To participate, you'll need to meet the following requirements:

* Obtain the Contest Pass: You get this from the Contest Coordinator in Slateport City.

* Have a Pokémon that meets the Contest requirements: Each contest has different types that are favored. You'll need to check the requirements for the Slateport contest.

* Have enough money to pay the entry fee: The entry fee varies depending on the contest.

Note: The Slateport Museum is not actually a museum in the traditional sense. It's a building that is used for Pokémon Contests.