Infinite SEO and custom post excerpt

Hi, the theme I am using is a directory theme and uses custom post types for the listings. I need infinite seo to pick up the excerpt and use it as the description meta tag, the default post setting is set to %excerpt% for the description but it is not picking it up for these post types. Because users will be adding items I do not want them to fill out the ultimate seo meta boxes. Any help would be much appreciated. Thanks.