Give it to the guy who asked you to get the goods back in rustboro city.
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To get the PokeNav, you have to deliver the Devon Goods to Devon, the scientist from Devon Corporation. Obtain these by beating the Team Magma Grunt in Rusturf Tunnel. Once you've done that, the president will give you the PokeNav.Note: This is for Ruby. I'm not sure if this applies to Emerald, Sapphire, or anything else.
You can find Devon in Mossdeep City he will give a Beldum. Note: Also works for Ruby and Sapphire.
Devon Corp. is in Rustburo city, west of the Pokemon gym and North of the house with 2 kids and a mother on the 2nd floor.
you can meet him after you get the devons goods back to the guy then he will bring you to meet mr. stone,the president of Devon coperation.mr stone will ask you a favour and give you a pokenav too
beat the 2nd gym and go in the cave