How to add an admin alert for missing plugins

To check if a plugin is installed and activated, use the is_plugin_active() function.

Note that you need to include this function first on the front-end before you can use it:

include_once ABSPATH.'wp-admin/includes/plugin.php';

Avoid function_exists() for doing the check. Plugins may refactor their code and a previously existing function might disappear. You never know.