Update: I’ve mixed up the “Product Catalog Feed Pro” plugin with the “Product Feed PRO” plugin, sorry about that.
For the “Product Catalog Feed Pro” plugin – it looks like this plugin doesn’t “like” the leading underscore symbol in our field’s name, i.e., _alg_ean
. So you must set it to something without an underscore, e.g., alg_ean
. This can be done in “WooCommerce > Settings > EAN > Advanced > Meta key”. Then you should be able to select it in the “Product Catalog Feed Pro” plugin settings:

P.S. If you’ve already entered EANs for your products, they will disappear after you change the meta key to alg_ean
. To bring them back, you can use our “Copy from product meta” tool in “WooCommerce > Settings > EAN > Tools” – set the “Meta key” option to _alg_ean
there and run the tool.
P.P.S. The “Product Catalog Feed Pro” plugin gets its custom fields list automatically by extracting meta keys from your products, so you need to set EAN for at least one of your products for the field to show up in their list. Also, the “Product Catalog Feed Pro” plugin saves the custom fields list in a transient with 24 hour expiration, which means that you need to either wait or delete the transient with this PHP code:
delete_transient( 'products_meta_keys' );