The unimod plugin is (not yet) part of the gentoo portage tree. To install it you will need to use a custom ebuild.
Download the attached ebuild archive and extract it to your local portage overlay directory. If you are missing an portage overlay add it to your make.conf
mkdir /usr/local/portage echo 'PORTDIR_OVERLAY="/usr/local/portage"' >> /etc/make.conf
now you can install it using
ACCEPT_KEYWORDS=~x86 emerge eclipse-unimod-bin