A Pokemon can't "gain" a hidden ability. A Pokemon has to be born or caught with its hidden ability.
E.g. you want a Sylveon with Pixilate. In that case, you must have an Eevee with Anticipation and evolve it into a Sylveon, whose ability will then turn into Pixilate.
To do this, either need to:
i) get an Eevee from a friend safari that has Anticipation
ii) transfer an Eevee from Gen V that had Anticipation
iii) trade for an Eevee that has Anticipation.
In order to breed hidden abilities, the parent must have the hidden ability already in order for the offspring to have a chance of inheriting it (it's not 100% that the offspring will inherit the hidden ability). There are 2 possible ways to do this:
i) have a male & female pair and the female must have her hidden ability
ii) have a male and Ditto pair and the male must have his hidden ability.
