All decorations received as a reward are automatically added to the Warehouse. To display a new decoration in the City you should:

1. Open the Building Gallery by clicking on the crossed tools icon in the lower-left corner of the screen in the City.
2. In the window that pops up, choose the Warehouse tab.
3. Select the decoration you want from the list and click on its icon.
4. In the mode that opens, move the decoration to the place you like and click on the checkmark.