If you used the Riverview_lite to build your world on, the game recognizes it as already installed. There's a very easy fix. You use a free program called s3pe, Sims 3 Package Editor) to open up your .world file, before you export it, and delete a file named WPID. The WPID file is like a product code, if two worlds have the same code, the game thinks they are the same.
I wrote a very easy tutorial on how to do it, just don't let the program intimidate you. I was scared to use it at first, but once I took the plunge, I was using it all the time. You can recolor the rabittholes, and trees, fun stuff!
Here's the tut, and let me know if you need any clarification on anything. :D
http://customsims3.com/forums/showthread.php?tid=102&pid=347#pid347
