You would need to transfer it to a B2/W2 game, then find the Move Tutor in Humilau City. Talk to him, and scroll down. You'll see the move Giga Drain. If you have at least 10 Yellow Shards, select the move. Then your party will show up. When you see Whimsicott, it will say "ABLE!". THEN it will know the move
Source: Bulbapedia