If you already have Carcassonne for iOS, then you'll need to download the update (big one it seems... took a full 2 minutes or so for me).
Then you'll be given the option to purchase Inns & Cathedrals for $2, and The River II for $1. *
Info on expansion related material...
Can I play against my familiy or friends by passing the device around?
Yes, of course. Up to five players can play on the same device.
Do I need to buy the expansions to participate in a game with expansions?
If you are playing local network multiplayer you can share an expansion with your opponents. To play online multiplayer, you need to buy the expansion to play with it.
• In-App store debuting with two expansions
• "The River" adds a new strategic element to the game play
• "Inns and Cathedrals" adds a double follower and tiles to upvalue cities and roads.
• Option to play a simplified version without fields. (Great for Kids)
• Option to disable tile overview and display of impossible placements
• Option to pick and chose expansions, if available
• Multiple bug fixes
* When I try to run it after the update, it just forces me back to the home screen, so I'm assuming that's how it works based on other iOS app expansions.