To get a Monster on Celestial Island, purchase a dormant Celestial from the Market using Keys. You must then revive it by zapping the required Eggs to its Inventory. 

Once you zap the first Egg, the timer to revive it will begin. If the Inventory is not filled before the timer runs out, the Eggs will expire. You will need to try again, but you will be compensated for the Eggs lost.