If the “Custom Fields” checkbox is not displayed under “Screen Options” in your WordPress Admin Area, there could be something in your current theme or a third-party plugin that is causing a conflict or perhaps disabling this setting as this functionality is included in WordPress by default.
Here are some things that you can try:
- Switch to a default theme temporarily and check to see if the “Custom Fields” option becomes available
- Deactivate all your plugins except FooEvents and WooCommerce to check to see if you still experience this problem. If not, you can reactivate your plugins one-by-one to see which one is causing the conflict.