deltafactory

Can you provide some more details and perhaps a list of what you’ve been trying and/or sample code?

When are you calling those functions? It should be after the right routines are loaded but before the page output begins. Ideally, use something like:

add_action( 'admin_menu', 'myplugin_menus' );
function myplugin_menus() {
  add_menu_page( ... ); //Fill proper arguments in..
  add_submenu_page( ... ); //Fill proper arguments in..
}

If you are trying to extend another plugin’s menu, make sure you’re creating your submenus after they create theirs. Try using a lower priority (higher number..) for the admin_menu action.

Finally, turn on debug mode so that you can see every little thing that’s going on. Good luck.

If they use wp_mail() as their sending routine, perhaps you can pair it with a good replacement that uses your SMTP settings. There also may be some options in your PHP config that need to be fixed.

WordPress doesn’t need to be held accountable for unexpected conflicts introduced by two third-parties including functions with the same name in their libraries. The official plugin development documentation strongly suggests keeping components in their own namespace to prevent conflicts like this.

Deactivate one of the plugins referenced in the error message, probably WP E Commerce.

From their changelog, it looks like you can use an older version (pre-3.8.4) without the mentioned conflict. Consider filing a bug with both plugin developers.