Are you talking about the English or the Japanese version?
If English, all you have to do is use mystery gift and choose recive gift via serial code. Once you have done that, you need to type in the shared code, which everyone can use once, PIKACHU20.
But remember, you can only get 1 of the pikachu, you cannot get all of them. Original pikachu is already over. The table is down below.
? September 19?25: Ash's Pikachu (Original Cap)
? September 26 ? October 2: Ash's Pikachu (Hoenn Cap)
? October 3?9: Ash's Pikachu (Sinnoh Cap)
? October 10?16: Ash's Pikachu (Unova Cap)
? October 17?23: Ash's Pikachu (Kalos Cap)
? October 24?30: Ash's Pikachu (Alola Cap