You need to be on the TouchWiz home screen when connecting to Kies. LauncherPro or any other launcher can't be in the foreground.

 

I use "Home Switcher." It's faster than having to go into application settings and then clearing the defaults for LauncherPro and then hitting the home key to be able to select the TouchWiz Launcher. Also, it's less error-prone because if you use the manual method that I just described, you could easily end up having to reinstall LauncherPro because you could accidentally click on "make this default" and select TouchWiz and have no native way to switch back to LauncherPro because there is no native way to clear defaults for TouchWiz.