i guess more people will need these instructions, so:
you need to add pro-audio overlay.
emerge -v layman
layman -kfa pro-audio
echo "source /usr/portage/local/layman/make.conf" >> /etc/make.conf
emerge -v csound
if you have an error complaining about opcodes is just because csound plugins are installed in /usr/lib/csound/ rather than /usr/local/lib/csound/ so you may have to set the OPCODEDIR to that.
happy csounding... :)
PS. also posted here