The Monster Units are unlocked by beating each Monster's special missions. These missions are available as soon as you unlock the island area which is home of the particular monster. Each mission can be played as many times as you want until you manage to beat it. When a mission was beaten, the corresponding island will become a regular island which can be taken over by other players.
To unlock the Medusa, you must discover the Gorgon Reef area and beat all 5 Medusa missions.