You re-installed the game and would like to restore your expansions (iPad or iPhone)
First of all, make sure that you are using the same Apple ID as before. Apps and expansions are tied to your Apple ID, so it is important to use the right one. This can be verified by going to Settings » Store: your Apple ID will be displayed there and you can switch to another one if you need to.
Ticket to Ride users: we also strongly advise that you connect with your Days of Wonder account in the app. Since version 2.1, this will save a backup copy of your expansions activation in your Days of Wonder account, which will avoid headaches in case of problems with the App Store. To do so, go to the Settings screen (in the Station using the "More" button from the main screen) and push the Login button. Then follow the instructions.
Restoring your expansions:
1) Start your Days of Wonder game and find the "Restore Purchases" button:
Ticket to Ride: when changing map
Ticket to Ride: in the Shop in the Station
Small World 2
2) Push the "Restore Purchases" button and enter your Apple ID password. This will restore your expansions.