I'm not sure how much this'll help, but add_sources is defined by us, in this file:
and called within each sub-folder of the project to add source files to the list to later be compiled/linked.
I've tried a couple of builds using cmake-gui, but can't get the same error. Using 'Codeblocks - MinGW Makefiles', and 'specify toolchain file for cross-compiling' as options, and the default 'C:/MinGW/cmake/toolchain.cmake' toolchain works just fine without any other fiddling. Attempting the same with Visual Studio 10 (I don't have 12 installed) returns an error about not finding ogre, nothing about add_sources. That may be related, as the FindOgre.cmake used is the default on in the CMake directory, not the one we provide in the projects cmake_modules folder, so for whatever reason that folder might not be being detected.
Sorry I don't know how to fix that, but it may get you a little further. Note that I looked into building with visual studio when I first set up cmake, and found it to be far from simple, eventually giving up and settling for codeblocks (or netbeans).