Monster mash blocks can be received by mining blocks. Monster mash blocks along with other Kewberno event items can be obtaind the same way you obtain cubits from mining, meaning, every time you mine blocks there is a chance of getting one of the blocks from this event. After you have collected all 5 Kewberno Machine blocks you need to place them accordingly. Before usage, a mash block must be placed over a receptor. Once you got that done, click use and the machine will generate one of the new mutant pets.