Unfortunately it seems that the Civ5 SDK only comes with the Steam version of the game. At least that explains why I couldn't find the Worldbuilder when I looked for it. *snip*
I'm not 100%, but I don't think you have to have the Steam version. If yours is boxed retail, you can install the Steam application, set up an account and you should be able to install the SDK.
I have the boxed version, but also have Steam for some other games, and this is what I did.
Let me know if you want me to get you detailed information, this is just off the top of my head!