Target opponent chooses 'land' or 'nonland'. Search your library for a card, exile it face down then shuffle your library. Any opponent may say 'challenge'. If they don't, put the card in your hand. If an opponent does, reveal the card. If it's of the initially specified type, the challenging player loses 5 life and you put the card in your hand. Otherwise, the card remains exiled.
DWIM (Play this card as if it had the wording it's obviously supposed to have)
Some of this wordiness can be omitted by not having the players say 'land' or 'nonland' ahead of time, and removing the exiled forever clause. Players will know, often, by looking at the game state, whether or not their opponent would want to tutor for a land. An easier way to write the card would end up:
Search your library for a card, exile it face down, then shuffle your library. Target opponent says 'land' or 'nonland'. Reveal the exiled card, then put it in your hand. If the revealed card is a land and your opponent said 'land', or if the revealed card is a non-land card and your opponent said 'non-land', lose 5 life.
Hmm... now I think I understand the problem. You want to be able to tutor for any card, and put the onus on your opponent as to whether they should challenge your lying activities... much like playing 'Qwijibo' in Scrabble. How about this version instead?
Search your library for a card, exile it face down, then shuffle your library. You may put that card in your hand unless any opponent says 'non-land'. If they do, reveal the card. If the card is a land card, put the card in your hand, and the player that said 'non-land' loses 5 life.
I know the intention is to allow players to play this card in mono-black, but man, this would read better if your opponent was forced to name a color, and if they were wrong they lose life...