Hi,
Please open this file Moodle/admin/settings/appearance.php and check for this code(line #162).
// Navigation settings
$temp = new admin_settingpage('navigation', new lang_string('navigation'));
$choices = array(
HOMEPAGE_SITE => new lang_string('site'),
HOMEPAGE_MY => new lang_string('mymoodle', 'admin'),
HOMEPAGE_USER => new lang_string('userpreference', 'admin')
);
is HOMEPAGE_MY choice is there ?