Incorporating the Settings API in WordPress Themes – by Chip Bennet

You should include your options-register-defaults.php to make the callback available.

function chr_settings() {

require_once 'options-register-defaults.php'; // add path to file

    /*  register_setting( $option_group, $option_name, $sanitize_callback )- Associates an option group passed to settings_fields with database entry */
    register_setting( 'mycustom_options', 'mycustom_options', 'ch_options_validate' ); 
    add_settings_section('ch_settings_defaults_style', 'Style Options', 'ch_settings_defaults_style_section_text', 'mycustom');

}