To add this overlay, edit /etc/layman/layman.cfg and add the following url to the overlays value https://raw.github.com/bjorhn/gentoo-overlay/master/profiles/repo.xml
Example:
overlays : http://www.gentoo.org/proj/en/overlays/repositories.xml
https://raw.github.com/bjorhn/gentoo-overlay/master/profiles/repo.xml
Then add it with layman -a oz