You get to the space center when you enter mossodeep city. for marage island you need to bring a pokemon from the island to a man in pacifiglog town. you talk to him and he will tell you if he can see it. you dont really need to go there all that there is are just whynut and some berries. so dont bother
DO BOTHER
It contains the rare Liechi Berry, which raises attack (I think) in battle when less than 1/3 HP.
It's also the berry used to make GOLD Pokeblocks, the best for any contest!
Make sure you plant it when you get it so you can get hundreds