Hello,
I am glad to find that you and I have similar experience. I came to know wxFormBuilder (wxFB) just one month ago, so I would like to share what I know with you.
wxFormBuilder is not a plug-in, but a stand-alone application. You should first download and install it:
http://sourceforge.net/projects/wxformbuilder/?source=directoryAs oBFusCATed said, wxFormBuilder is a code-generating tool. The wxFB project file ends with .fbp, which you can find in the project created by CodeBlocks wizard. You should open it with wxFB (which might have been the default application for that type in Windows after the installation). Then you will see the similar GUI designer.
Generally, wxFB is used to create a base class for all GUI-related codes and generate a derived class to be modified for action and response. Once the apperance is modified, the base class should be re-generated. Even if the derived class is to be updated, its contents will be kept. The major difference between wxFB and wxSmith may be that wxFB separates action code from the user interface.
When you finish the code, just build and run it in CodeBlocks as you use wxSmith.