Choose one of your opponent's Creatures in any arena. This turn, you choose whether or not that unit attacks and which unit, yours or your opponent's, it attacks.
If this is played on an opponent's Creature unit, and that unit has an ability that triggers "when this unit would attack," then your opponent may play that ability instead, overriding the effect of Tame the Beast.
The defending unit you choose must be in the same arena as the attacking Creature.