Make sure you have defined the OSG_FILE_PATH
environment variable and that it points to the data directory (the one with 'cow.osg').
You can then set OSG_NOTIFY_LEVEL
to DEBUG_INFO
to get extended information printed to the console. If for any reason the file could not be loaded correctly you will see where to start to troubleshoot.
You can also try to run
$ osgviewer --image /path/to/an/image.png
to test if the plugins work correctly.
Plugins are named e.g. osgdb_jpeg.dylib
so you can look for these to make sure they have been built properly along with the core libraries and programs.