I couldn't easily or intuitively figure it out in the app, and I just happened to stumble on the fix...
- Login to facebook.com on a computer
- Point your mouse over your "Interest" you want to add in the navigation on the left hand side.
- click the pencil to the left and "add to favorites".
- It will add to the "Favorites" section of your navigation.
- Now check your iPhone and it's there too!