How to use sdpm() or dpm()?
hook_menu_link_alter() accepts only one argument in Drupal 7. That's why your code isn't working. 

Try the following:

function YOURMODULE_menu_link_alter(&amp;$item) {
  if ($item['menu_name'] == 'YOUR_MENU') {
    dpm($item);
  }
}


I have wrapped it in an if check, since otherwise it would run dpm for all menu links on a page and depending on the amount of links it may cause memory overflow or timeout.