This was taken from the WordPress Codex reference for is_plugin_active
In the Admin Area:
In the front end of a theme you need to include a reference to the following library
include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
In this case $plugin is the directory of the plugin followed by the main php file. In this example below the event Event Espresso plugin directory is used.
$plugin = event-espresso/espresso.php