emerge layman (if you don't have it yet)

Edit /etc/layman/layman.cfg and add this: (second line)

overlays  : http://www.gentoo.org/proj/en/overlays/layman-global.txt
	    http://github.com/castiel/gentoo-overlay/raw/master/overlay.xml

Then:
layman -L
layman -a castiel

then you can emerge anything from the castiel overlay