As long as MeCat is not yet approved by the gentoo administrators, you need to download the ebuilds manually from
bug 109265 into the portage overlay folder. The name of the portage overlay folder can be found
in the /etc/make.conf file. As an example, assume the portage overlay folder is /usr/local/portage. Then you have to download the file mecat-1.4.2.ebuild to /usr/local/portage/app-misc/mecat/mecat-1.4.2.ebuild.
Once all that is done, you have to go into the directory /usr/local/portage/app-misc/mecat/ and run
ebuild mecat-1.4.2.ebuild digest
After that, you can emerge MeCat like every other development package, i.e. depending on the architecture, it can be installed with the keyword ~x86, ~amd64, ~ppc, etc.
ACCEPT_KEYWORDS="~amd64" emerge mecat
MeCat requires Java 5.0 and at the time I wrote this, Java 5.0 was still marked unstable.
A guide for installing Java 5.0 can be found at http://www.gentoo.org/proj/en/java/java-upgrade.xml.
Remark: One problem that may occur is that MeCat will not accept special characters.
The reason for this misbehavior lies with the language settings of gentoo (or the lack of those).
A description on how to setup the system correctly can be found at: