The "additional castle elixir" or the "Bric-a-Brac" (not sure if spelled correctly) elixir?
If you got it from a bundle it should be inside a little giftbox type thing to the left of your screen. The "Additional castle elixir" allows you to add one to the max number of houses you can buy. The max without one is three but using it you can add on the amount of houses you can have. The "Bric-a-brac" extends the amount of items you can have either inside or outside of a castle. Both have an instant effect when equipped!
Hope that helped: Isaac Moonshard, level 86, Necromancer