You're going to install a 3rd party  ebuild, that is currently not in
the Portage tree. The package was proposed to the Gentoo Portage Team
and we are hoping to see it in the official portage soon.

You can see the complete intructions at this page:

http://gentoo-wiki.com/HOWTO_Installing_3rd_Party_Ebuilds

Setting Up Portage Overlay

The first thing you should do is setup a PORTDIR_OVERLAY directory, you
will need to add this line to your /etc/make.conf:

File: /etc/make.conf

PORTDIR_OVERLAY="/usr/local/portage"

This directive will allow you to install your custom ebuilds in a way
that will not be affected by emerge sync.

Placing the Ebuilds

First check that /usr/local/portage exists; if it does not, create it.
You can use:

# mkdir -p /usr/local/portage

When placing the ebuild in /usr/local/portage, you must keep a similar
structure to that of /usr/portage
(category/program/program-version.ebuild).

The category part must match one of the lines
in /usr/portage/profiles/categories. In this case, if you want to
install KLinpopup you would create:

# mkdir -p /usr/local/portage/net-im

Also make sure that program part of the directory name matches the name
of the ebuild without the version number. In this case your ebuild is
called klinpopup-0.3.4.ebuild, then you need to place this file in the
directory /usr/local/portage/net-im/klinpopup/

Installing using the new ebuild

When you go to install the package using the new ebuild, it will most
likely be masked by keyword, so you should use the method of unmasking
by keyword that is described in man portage. Currently we do this by
opening the following /etc/portage/package.keywords file in your
favorite text editor. Add the following line at the end of the file,
replacing <category> with the category the new ebuild is in (e.g.
app-portage), <package> with the name of the package (e.g. klinpopup).
File: /etc/portage/package.keywords

net-im/klinpopup

Now enter the package directory in the local portage:
# cd /usr/local/portage/net-im/klinpopup
and run:

# ebuild klinpopup-0.3.4.ebuild digest

now you can run emerge -pv to see if everything goes well:

# emerge -pv klinpopup

Portage will tell you that a portage overlay is been used:

 Portage overlays:
   [1] /usr/local/portage

   For any kind of question, just send me an email to digital [dot] death
   [at] gmx [dot] it