First of all, I don't think to set the store country to Belgium you need to use the functions.php. Please make sure that you have the latest version of the Pro which is 2.0.4, if not please back up the website and update the theme.
Then please make sure that you use the version 3.3.3 of the Woocommerce plugin, if not please update that plugin. Then go to Woocommerce > Settings > Shipping > Add Shipping Zone and set the Country to be Belgium:
That should do the trick. But in any case, if you need to add the code to the functions.php you do need a Child Theme. Please read the article below to know how to install the Child Theme:
After the installation, you can simply go to Appearance > Editor > Theme Functions and there you can access the functions.php file and add whatever code you like:
Installing and activating the Child Theme will not affect your settings and additional CSS in the Theme Options.