You get it in Slateport City, when you are about to leave. After completing the town story line with Team Aqua (Or Magma) All you have to do is begin to leave the town through the north exit. Before you head out, there will be some sort of commotion, a super star is standing outside of the contest hall and you will be "scouted" by her. She will give you a Contest Pass, as well as a Pokeblock Kit. Hope This Helps! Sorry that I don't know more.
*Metagross2097 AKA TheMysticalTaco*