Alter the menu_edit_item form
That code should work in both in Drupal 6, and Drupal 7.

If you edit an already enabled module, you should clear the Drupal cache, as the list of hooks implemented by modules is cached by Drupal; that means that changing the code of an enabled module doesn't cause Drupal to notice the new added hook.

As alternative, you can go to the page listing all the enabled modules (admin/modules), disable your module, click on the "Save configuration" button you find at the bottom of the page, re-enable your module, and click again on the the "Save configuration" button. This will force Drupal to clear the cache where it saves the list of hooks implemented by the enabled modules.