Holy Potatoes! A Weapon Shop?!: Launch the game, load your save file or start a new game, click on the Settings icon at the bottom right, and you can enter the Promo Codes. Holy Potatoes! We’re In Space?!: Launch the game, click on Settings and you will see Promo Codes bar to key in.

Those codes are requires to unlock the achivement "Interior Designer" where you need to unlock all the decorations.