How to alter add content form to include a new setting panel?
Your code works perfectly for me; dropped straight into a form alter hook I get the following on the article add page:



There are 3 reasons I can think of that might explain why you're not seeing the new tab:


You haven't cleared Drupal's caches since implementing the new hook (probably the most common cause for problems like this).
Your module isn't actually called "mltag", in which case your form alter hook will never actually be run.
Your code is in a file other than the main .module file. Hooks declared in files that are not explicitly included from the .module file will not necessarily be found when Drupal is updating the module_implements cache.