% babel -client=C file.sidlor more cryptically
% babel -cC file.sidl
This will create more files than you can shake a stick at. The files ending in _IOR.h and _IOR.c are the Intermediate Object Representation. The files ending with _Stub.c are the C stubs -- the interface between a C client and the IOR. The remaining header files have external C API that C clients may use.
To use the C stubs, you must compile the stub files whose file names end with _Stub.c and link them against the SIDL runtime library and a backend implementation.