The obabel command line executable has a bug on the Mac OS X operating system. If both the input and output format is CML then the conversion fails. This functions as expected on Windows and Linux, and does not appear to pose a problem for other formats, such as SDF and XYZ, on Mac OS X. This affected both the last release (2.3.2) and the tip of master using the latest Snow Leopard patched release and build with GCC 4.2 that comes as default.
obabel -icml -ocml --minimize was one command line we tried, changing either input or output format (or both) allowed the operation to be processed as expected.
This seems to work fine on current git master and Mavericks. Can you still reproduce?