Do you have the new field included in the database table?
Do you have the sql statements necessary to read and write the information? (includes/modules/pages/links_submit/header_php.php)
Have you included the necessary error catching routines? includes/modules/pages/links_submit/jscript_main.php)
Have you included the necessary code on the submit link page?
Have you included the necessary sql statements to read/write the information?
Have you added the necessary code to display the new field? (admin/links.php)