This example below will add the WordPress Media button to all Rich Text fields used throughout your site. Please note that the Add Media button will only appear for users who have permission to edit WordPress posts.

// Add "Add Media" button to TinyMCE editor
add_filter('frm_rte_options', 'frm_rte_options', 10, 2);
function frm_rte_options($opts, $field){
    $opts['media_buttons'] = true;
    return $opts;
}

If you see the Sorry, you are not allowed to attach files to this post error message, check if your users have the correct capability to edit_others_pages and edit_published_pages.

This variation on the example above will add the WordPress Add Media button to all Rich Text fields throughout your site, but only for Administrators.

// Add "Add Media" button to TinyMCE editor
add_filter('frm_rte_options', 'frm_rte_options', 10, 2);
function frm_rte_options($opts, $field){
  if ( current_user_can('manage_options' ) ) {
    $opts['media_buttons'] = true;
  }
  return $opts;
}

Use this code example to limit the TinyMCE Editor options to all Rich Text fields use in the site.

add_filter('frm_rte_options', 'rte_limit_options', 10, 2);
function rte_limit_options($opts, $field){
	$opts['tinymce'] = array("toolbar1" => "undo redo bold italic alignleft aligncenter alignright alignjustify | bullist numlist outdent indent");	 	
	return $opts;
}

See the available options in the TinyMCE documentation.