how to make separate field in wordpress user_data_

This is the serialized string. You need to unserialize it:

$raw = get_user_meta($current_user->ID,"Tw941I8fXg_s2member_custom_fields",true);
$data = maybe_unserialize($raw);
extract($data);

Now, $data will have an array of all options, and the extract will turn each element into a variable with a value.