To choose a random creature from a random graveyard, first choose a random graveyard with at least one creature card in it. Then choose a random creature card from that graveyard.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

The choices of random creature and random graveyard are made as Call from the Grave resolves.

Playtest cards use a modified version of game symbols, such as and . These modified symbols should be treated as the standard symbols during play.

The amount of damage Call from the Grave deals is determined by the casting cost . . . er, mana value of the permanent that was put onto the battlefield by Call from the Grave (even if it’s not a creature at that time). In the case where no permanent was put onto the battlefield, Call from the Grave won’t deal any damage.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).