PHP Error messages

I’m getting various error messages caused by this plugin:

Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method plugin_simple_add_pages_or_posts::addPluginSubMenu() should not be called statically in /Users/website/public/wordpress/wp-includes/plugin.php on line 525

Warning: Missing argument 2 for plugin_simple_add_pages_or_posts::addPluginSubMenu() in /Users/website/public/wp-content/plugins/simple-add-pages-or-posts/simple_add_pages_or_posts.php on line 47

Warning: Missing argument 3 for plugin_simple_add_pages_or_posts::addPluginSubMenu() in /Users/website/public/wp-content/plugins/simple-add-pages-or-posts/simple_add_pages_or_posts.php on line 47

Strict Standards: Non-static method mijnpress_plugin_framework::addPluginSubMenu() should not be called statically in /Users/website/public/wp-content/plugins/simple-add-pages-or-posts/simple_add_pages_or_posts.php on line 50

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /Users/website/public/wordpress/wp-includes/functions.php on line 3842

Warning: Cannot modify header information - headers already sent by (output started at /Users/website/public/wordpress/wp-includes/plugin.php:525) in /Users/website/public/wordpress/wp-includes/option.php on line 823

Warning: Cannot modify header information - headers already sent by (output started at /Users/website/public/wordpress/wp-includes/plugin.php:525) in /Users/website/public/wordpress/wp-includes/option.php on line 824

Strict Standards: Non-static method mijnpress_plugin_framework::addPluginContent() should not be called statically in /Users/website/public/wp-content/plugins/simple-add-pages-or-posts/simple_add_pages_or_posts.php on line 58