As The Mimeoplasm enters the battlefield, you may exile two creature cards from graveyards. If you do, it enters the battlefield as a copy of one of those cards with a number of additional +1/+1 counters on it equal to the power of the other card.
9/22/2011 You can’t choose to exile just one creature card.
9/22/2011 The two creature cards may come from the same graveyard or different graveyards.
9/22/2011 You choose which one The Mimeoplasm is copying and which one determines how many +1/+1 counters are placed on The Mimeoplasm as The Mimeoplasm enters the battlefield.
9/22/2011 Treat The Mimeoplasm as though it were the creature card it’s copying entering the battlefield. Any “As [this card] enters the battlefield,” “[This card] enters the battlefield with,” and “When [this card] enters the battlefield” abilities of that creature card will work.
9/22/2011 Specifically, if The Mimeoplasm enters the battlefield as a copy of a creature card that enters the battlefield with a number of +1/+1 counters, The Mimeoplasm will enter the battlefield with those +1/+1 counters and +1/+1 counters equal to the power of the other exiled card.