for my current theme( own custom ) the default menu options are disabled in wp-admin?

Before you can use custom menu’s, you have to add one. Try adding the menu by typing a name and clicking save. If you don’t have custom menu’s in the theme files, you should still be able to use the custom menu widget assuming you have enabled them in your functions.php file. If you don’t have either, just comment here and I’ll explain how to add them.