There is a new section at BabaCAD homepage, Developers section: http://www.babacad.com/developers
You can visit Developers section to learn how to write your own extension modules for BabaCAD (.bem). It’s very easy to share .bem modules between users, as .bem files just have to be copied to BabaCAD/ExtensionModules folder (BabaCAD install folder is in \Program Files\BabaCAD directory). BabaCAD application will automatically load all .bem modules from this folder at the start time.
LISP module, which is the first developed .bem module for BabaCAD, shows many features programmed to extend functionality of BabaCAD software. Microsoft Visual Studio 2010 is only requirement to start programming applications for BabaCAD. BabaCAD v1.3 installs include headers and lib files of BabaCAD Extension API. It can be found in \Program Files\BabaCAD\BabaCAD\API directory.
Read Developers section and you will find example of SLOPE command inside Civil Tools extension for BabaCAD. Full visual studio c++ project (.sln) is there (in zip file). I recommend everyone to use this as a template for theirs new .bem projects. There is necessary entrypoint function of extension API and also some other API functions (for example AddCommand function adds new command to BabaCAD runtime).
I will try to add more examples. Also, I will add a new topic at BabaCAD forum for developers to ask questions and submit comments.