How to call value on single.php?

I’m able to call variables on archive/category pages, but not on single posts. Here is the code that is working, what do I need to edit to get this to call the category on single posts?

<?php
$cat_ID = get_query_var(‘cat’);
$url = categoryCustomFields_GetCategoryCustomField($cat_ID, ‘Twitter’);
$url = explode(‘@’,$url[0]->field_value);
$url = $url[0];

echo $url;
?>

http://wordpress.org/extend/plugins/categorycustomfields/