The Codex page for is_plugin_active explains what your problem is:
NOTE: defined in wp-admin/includes/plugin.php, so this is only available from within the admin pages. If you want to use this function from within a template, you will need to manually require plugin.php, like this:
<?php include_once( ABSPATH . 'wp-admin/includes/plugin.php' ); ?>
<?php is_plugin_active($plugin) ?>