If you were given a code by a friend to play Hotel Hideaway, once you reach level 8 you will be able to redeem that code for a gift. You will only be able to do this once. 

Upon reaching level 8, you will be sent a pop up in order to redeem the gift. When you enter the screen to get your present, simply type the code into the text box, and then tap the redeem button. 

If it's not working for you, here is a few things to check:

• Was your avatar created after the code was created? If not, your avatar is not eligible for the gift.
• Have you already redeemed an invitation code? You are only able to use it once per avatar.
• Has the person that gave you the invitation already reached tier 3 or the invitation achievement? If so, they are no longer able to give out code.
• Is it another avatar on your account? An avatar on one account cannot use an invitation from another avatar on the same account. 

If you dismiss the  box before you have a chance to redeem the code, don't worry, you will still be able to open the gist in the 'Redeem a Gift' box of the meta shop (where you buy diamonds.)