Calling the compiler without arguments will show you the command line options it supports:

$..> mgen

    *********************************************************************
    **                                                                 **
    **                                                                 **
    **        MGen Compiler (SNAPSHOT 2014-08-31 16:58:56 +0200)       **
    **                                                                 **
    *********************************************************************

Valid MGen compiler arguments are:
  -help: displays this help
  -project="myProjectFile.xml": specify project file (Required)
  -plugin_paths="my/external/path1, my/external/path2": specify additional plugin paths (Optional)
  -output_path="specify output path (Optional)
  -fail_on_missing_generator=true/false: Default false (Optional)
  -check_conflicts="true/false" (default=true): If false: the compiler will ignore any type name/id/hash conflicts (Opti onal). Useful for IDL<->IDL translation
  -include_paths="/home/me,/home/you": The paths to search for files

Environmental variables can also be used:
  MGEN_PLUGIN_PATHS: See command line argument 'plugin_paths'
  MGEN_INCLUDE_PATHS: See command line argument 'include_paths'
  MGEN_INSTALL_PATH: if set, ${MGEN_INSTALL_PATH}/jars will be searched for plugins