$post_id = 25;
$meta_key = 'wpsnipp_post_extra_resource_links';
// Add one resource
$meta_value = 'http://google.com';
add_post_meta( $post_id, $meta_key, $meta_value );

// And another
$meta_value = 'http://amazon.com';
add_post_meta( $post_id, $meta_key, $meta_value );

// No resource list would be complete without wpsnipp.com
$meta_value = 'http://wpsnipp.com';
add_post_meta( $post_id, $meta_key, $meta_value );