For anyone else who finds this, the wc-add-to-cart-variation
script is loaded by WooCommerce in the footer. So, if it isn't loading your theme either 1. doesn't have wp_footer()
(which is rare as even crap themes should have this) or 2. there is a PHP error in the theme templates and the page is not fully loading.
I would suggest checking the latter point. You can view a page's source code. In Chrome just type CTRL+U.
Scroll all the way to the bottom. The last thing you should see is:
</html>
If you don't see this the page didn't load fully, likely do to a fatal PHP error.
You can find out exactly where by enabling WP_DEBUG
in your wp-config.php
file.
If anything, I'd guess that your theme has some WooCommerce templates that are out of date with respect to the current WooCommerce. This post is old, but this keeps happening as WooCommerce keeps evolving and themes keep packaging templates that they don't even modify.
This issue is probably caused by an outdated single-product/add-to-cart/variable.php
template. But if renaming that doesn't solve it, in a pinch you can always disable your theme's entire WooCommerce templates by renaming the WooCommerce folder in your theme to anything else. 99% of the time it will be called woocommerce, but it could be something else.
WooCommerce will now let you know which templates are out of date. This information can be found in the admin under WooCommerce>System Settings.
Another good read would be:
http://develop.woothemes.com/woocommerce/2014/02/solving-common-issues-after-updating-to-woocommerce-2-1/