How to output a field as a link in D7
it sounds like you aren't trying to add a field, but rather, are trying to DISPLAY a field (link, more specifically) on the user account page.

What you need to do is simply edit the user profile template of your theme to show the link you need in the footer/header or wherever else you need.

Something like the below should work:


Go into our drupal root folder and from /modules/user/ copy the user-profile.tpl.php file and paste it into your theme's /templates folder
It should just have code that looks like this to begin with:



&lt;div class="profile"&lt;?php print $attributes; ?&gt;&gt;
   &lt;?php print render($user_profile); ?&gt;
&lt;/div&gt;



Add your link at the bottom, making the template file something like


&lt;div class="profile"&lt;?php print $attributes; ?&gt;&gt;
    &lt;?php print render($user_profile); ?&gt;
    &lt;?php if (in_array('my_role', $user-&gt;roles)): ?&gt;
       &lt;p&gt;&lt;a href="http://www.example.com"&gt;My link&lt;/a&gt;&lt;/p&gt;    
    &lt;?php endif; ?&gt;
   &lt;/div&gt;




Make sure you clear your caches, visit a user account page, and make sure it's all working.  
Adjust the HTML above as you see fit...