Ordering a list of posts by a custom date field

I am using custom post types (Events and Exhibits) created with CustomPress. Both post types have custom date fields (Event Date and Exhibit Opening Date). I need to create pages that list posts for each of those post types ordered by date field. Previously, before creating custom post types, I was using a custom field plugin and I was able to use this code to get the posts to show up in chronological order:

$args = array('post_type' => 'events', 'orderby' => 'meta_value', 'meta_key' => 'event-date', 'order' => 'DESC');

Now that I am using custom fields generated with CustomPress, this code (with the new custom field names) no longer works. It appears that the problem is that the CustomPress custom date fields store the date in month-date-year format, because my posts are now ordered according the number of the month. How can I fix this?

Thank you!