so now you know what to do
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go to mount pryre.
Go to the top of Mt. Pyre summit and you will discover Maxie/Archie already took the Orb that was upon the summit. The elderly man will give you the Magma Emblem to open Team Magma's hideout.
Go to Jagged Pass and go in the middle where the Magma guy was. There is an earthquake and a door appears.
POKEMON EMERALD (What the Pokedream guide misses out (and everyone who has answered so far)): You get a Team Magma Emblem at Mt Pyre. You then go to the Mt Chimney Chair lift and take it up. Once on top, go down the Jagged Pass. As you go down, the Magma Emblem will open up a cave for you to go into. Complete this before you are able to go to Slateport to meet Captain Stern.
In Emerald, an old lady in Mt. Pyre will give you Magma Emblem. After you got it, go to Judged Pass. If you bring the Magma Emblem, the Magma Hideout will appear in Judged Pass.