Enzo is a rather complex code. At the time of this writing, the code is composed of 413 C++ and fortran source and header files with a total of 86,245 total lines of text (found by doing running "wc -l *.C *.src *.h *.def" in the amr_mpi/src directory. Needless to say, modifying the code can be rather challening.
Unfortunately, we haven't had time to write the Developer's Guide yet. It's coming soon, and we would appreciate any user suggestions as to what we should include in the Guide. In the meantime, you may find the Enzo Flowchart and Code Browser handy.