Can Baron be chosen from hand to be discarded Mindsplitter?

Yes, Baron Nashor can be chosen and discarded from your hand by Mindsplitter's ability.

According to the card text for Baron Nashor, he has the ability: "I can't be chosen by enemy spells and abilities."

However, this protection only applies while Baron Nashor is on the battlefield (a unit). When Baron Nashor is in your hand, he is considered a card, not a unit, and his abilities are not active. Therefore, he does not have protection from being chosen by Mindsplitter's "When you play me" ability while in your hand.