Requires: 3.5 or higher
This assumes you are using WooCommerce 2, if you are using a version below 2 then you don't need to upgrade and it's going to be something else.
In either case you might be able to track the error down by enabling WordPress debug mode (http://codex.wordpress.org/Debugging_in_WordPress) and trying to add a product. There may also be entries in your server error log which can point to the problem.
If nothing shows up then a standard practise is to deactivate all other plugins and swap the theme to Twenty Twelve and try to add a product, if that works change your theme back and test again, then reactivate your plugins one by one, testing each time. If it stops working again then you will know where the issue is
I work for WooThemes