When Jace's Mindseeker enters the battlefield, target opponent puts the top five cards of their library into their graveyard. You may cast an instant or sorcery card from among them without paying its mana cost.
7/1/2013 If you cast an instant or sorcery card this way, you do so while the ability of Jace’s Mindseeker is resolving. If you choose not to (or can’t), you won’t get a chance to cast one later.
7/1/2013 “From among them” means the five cards put into the graveyard, not all cards in that graveyard.
7/1/2013 When casting an instant or sorcery card this way, ignore timing restrictions based on the card’s type. Other timing restrictions, such as “Cast [this spell] only during combat,” must be followed.
7/1/2013 If you cast a card “without paying its mana cost,” you can’t pay alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.
7/1/2013 If a card has in its mana cost, you must choose 0 as its value.
7/1/2013 If the five cards are put into a public zone such as exile instead of a graveyard (perhaps due to the ability of Rest in Peace), you can cast one of those instant or sorcery cards from that zone.