At the beginning of each player's draw step, that player exiles the top card of their library. If it's a land card, the player puts it onto the battlefield. Otherwise, the player casts it without paying its mana cost if able.
6/1/2011 Putting the land onto the battlefield or casting the card is not optional.
6/1/2011 If the player is unable to put the land onto the battlefield (this is rare) or cast the spell (perhaps because there are no legal targets available) as the triggered ability of Omen Machine resolves, the card simply remains in exile.