Change spawn point sims 3 create a world?

I'm kind of confused by what the problem is. Are you talking about when you go into Edit in Game mode for placing/editing buildings? The game will put you into live mode initially when you choose that option, but you need to click on the button for the Main Menu and choose Edit Town. Once you're in Edit Town, you can start placing and editing buildings on the lots you placed in CAW. The "spawning" point in Edit in Game mode shouldn't really affect anything.

And if the game is letting you go too far out on the map, the best solution for that is using the non-routable terrain paint and the non-routable camera paint in CAW. There should be options for both in the terrain sculpting/paint tab.

If you want to see an example of it, open up the Sunset Valley or Riverview Lite worlds in CAW, click on those paint options under Terrain, and you'll see yellow and blue paint on the terrain in those worlds. For the most part, non-routable terrain paint is used on the outer edges of the world and on any terrain where Sims are likely to get stuck if they try to walk on it (really steep sides of hills for example). The camera non-routable paint is used on the outer edges of the world so, for example, you don't have to deal with the camera drifting to the very edges of the world (letting you see the distant terrain up close, which can be pretty ugly).