The portal is in a circular room in one of the tallest towers of the Fairy Palace in the Fairy Kingdom. It holds an empty archway. Mother Goose and the others used to travel through this portal "in the good old days".
Bree and Emmerich travel through it at the end of A Grimm Warning.
Mother Goose doesn't remember exactly where the portal leads to on the other end. "Somewhere in the Netherlands. Or was it Nevada?"