Did you download WP-Cron 1.4 14 ?
http://www.skippy.net/blog/category/...ugins/wp-cron/
http://www.skippy.net/download/plugi...p-cron-1.4.zip
Also check the user notes
http://www.skippy.net/blog/2005/05/26/plugin-wp-cron/
I'm not sure why you want to ping every 15 mins most people ping hourly or daily. There is a lot of info on cron jobs also on wordpress.org