You need the aurora ticket which was given out on a Nintendo event which has long since passed.
You must obtain Deoxys from Birth Island in Emerald.
Birth island
Birth island.
Faraway island, Birth island and Southern island.
On Birth Island.
You go to birth island
the way to get to birth island is to get the auroua ticket
If you got the Birth Island event then you can get it.
Need a Aurora ticket to get there.
Nintendo event. Birth Island Ticket.
eat a potatoe
You can't go to the island unless you have the aurora ticket which is now impossible to get without cheats, with that show the sailor who takes you on the ships he will take you to Birth Island.