You need to have already been to every lake during the snatching of the mirage pokemon.
Then head to Veilstone and head through the Galactic HQ until you reach the Boss. Battle
him then he will give you the masterball.

First, make sure that u have been to the 3rd floor of the canalave library. then u have to
go to veilstone and talk to the person outside. he will give u the key. head inside and
battle cyrus, the team galactic boss. after u beat him, he will give u the masterball

u get one if you win grand prize