There's a bit more documentation coming installed with the DevKit. You can find some geometry stuff in the following subfolder: API Development Kit 25.3002/Documentation/HTML/Geometry/Geometry.html
Unfortunately it's a bit out-dated since it doesn't really document much of the newer interfaces which work more "C++ style" with classes etc. Best is to go through the headers in: API Development Kit 25.3002\Support\Modules\Geometry. There you also find the definition of CustomMultiPolygon2D in the file "Polygon2D.hpp". So including that header should get rid of the error.
Thank you for the suggestion! Including the above mentioned header file solved the building problem. But as it turnes out there is a long process ahead of me to understand how the function works!. I'll check the documentation and the example codes. Again, thank you!