Trying to have both OptimizePress and MarketPress co-exist

I'm having some trouble getting MarketPress to play nicely on a site with the OptimizePress 2 theme.

To me it looks like MarketPress is not using the template_include or index_template filter tags when it displays second level pages such as product lists for a specific category.

This page works:

This one does not:

A snippet from the OptimizePress code where they have a switch to make it compatible with WP e-commerce:

add_filter('template_include', 'op_template_include');
* External plugins (WP e-commerce) are skiping the 'template_include' hook
* and are instead loading theme "index.php" template which shows "Finish blog setup" screen
* Added a checkbox in Dashboard -- Global settings -- external plugin compatibility
* This works only if that checkbox is on! to deal with some issues
$val = op_get_option(' ');
if ('on' === $val) {
add_filter('index_template', 'op_template_include');

The page is not working because the op_template_include is not being loaded. What filter should I tie in to to get it to load on the product list page for a specific category?