Calling wordpress function from plugin?

I’m trying to call the function wp_create_category from a plugin file but I’m not having much luck with it as the function is undefined.

I tried to relatively include the taxonomy.php file from wp-admin (which is what contains the function) but the file is not found even though i’m sure I’ve got the right number of ‘../’s to get tot he root install directory before going ‘/wp-admin/includes/taxonomy.php’