Ajax Error with Marketpress - Category Menu and Add to Cart Button not working


I'm having some trouble with Marketpress. I have it installed on my multisite, and everything is working fine on my subdomain, but I'm having errors with my main site.

This is the error I'm getting.

XMLHttpRequest cannot load http://simplymusicteachers.com/wp-admin/admin-ajax.php. Origin http://www.simplymusicteachers.com is not allowed by Access-Control-Allow-Origin.

Any suggestions on how to fix this?