For an initial description of the data formats, please see http://darksky.slac.stanford.edu/data_release/ and https://bitbucket.org/darkskysims/sdf.
For examples reading SDF data in Fortran, C, or with the libSDF library, see http://bitbucket.org/darkskysims/sdf_example.
If you like python, you may want to check out yt, which has built-in readers for the SDF data format. The I/O routines that are in yt have also been packaged into https://pypi.python.org/pypi/sdfpy, which can be installed with `pip install sdfpy`.