Did you read the Sabayon Linux forum post I linked to?
In it, I provided examples of how to check for installed software, prepare to install software (decide what options you want), and install it, using real examples. That post also has links to the official Gentoo documentation on it, which is pretty good stuff, too.
If Kuroo is complaining when you start it up, do an "emerge --sync && emerge portage" as root to update your portage tree (list of available programs) and portage itself. Portage is essentially a gigantic repository of sources (as well as some binaries). When you run "emerge <package>", portage downloads the source from a Gentoo mirror, then checks what options you told it to compile with (you can change these using USE flags), and compiles and installs it. It also has standard places of putting things, so if you merge two packages that need each other, they will know where to find each other. It automatically handles dependencies.
Portage uses one file directly in /etc (/etc/make.conf) and several files in /etc/portage to determine settings.
make.conf:
Tells portage what to compile for, your optimizations, your global USE flags, your languages, and your overlays (a portage overlay is like a third-party extension to the portage tree).
In /etc/portage we have:
package.mask: portage will treat packages in this file as if they don't exist. This can be nice if you want to avoid upgrading to a buggy version of a package. For example Amarok 1.4.2+
was giving me trouble, so up until 1.4.3-r1 came out, I had ">=media-sound/amarok-1.4.2" in my /etc/portage/package.mask file.
package.unmask: same idea, but for unmasking packages. I have a few packages listed in mine, including "www-client/mozilla-firefox" and "net-im/gaim" in here.
package.use: a list of package specific USE flags.
Those are the only ones you really need to mess with.
Hope that helps!
--Patrick C.