If you go into your building menu, there are 6 tabs at the top of the menu, the 6th tab is expansions, once you click that it will show you 3 ways you can get them with the far right option being the one you are looking for and it should be highlighted that you have an expansion to place..
If it still is not showing as you having one available then try refreshing the game and checking.... If that still fails then I would send in an in game support ticket explaining your situation