The issue here is that the users info in the sandbox is not accessible as they are pseudo-users.
For the time being at least, you will have to take the id of the user and dynamically create the link yourself in your view/layout. Something like;
echo '<a href=index.php?option=com_... &id=' . $user_id . '>' . $user_name . '</a>';
The way that joomla articles handles this is by creating a link to a contact page for the author as opposed to a 'profile' page - in J2.5 at least.
Anyway, you get the principle... So you can redirect it to any page you like based on the authors user id.
Hope this helps,
Gez
