You don't have to choose a card to copy. If you don't, Activated Sleeper enters the battlefield as a 0/0 creature and is probably put into your graveyard immediately, unless something is increasing its toughness to keep it alive.

If the chosen creature card has in its mana cost, X is considered to be zero.

Treat Activated Sleeper as though it were the chosen card entering the battlefield, except it's also a Phyrexian. Any "As [this creature] enters the battlefield," "[This creature] enters the battlefield with," and "When [this creature] enters the battlefield" abilities of the chosen card will work.