Hook up the mobile adapter. Go to the communication center in Goldenrod City, and read the message on the lockers to the right of the room. They will talk about a present from the future. Leave and the blue-haired nurse will ask you to defeat all sixteen gyms in order to obtain the GS Ball. Once you have done so, give the GS Ball to Kurt. The next day, he will give it back. Put it in the shrine in the Ilex forest and a level 30 Serebii (Celebi) will attack you.