To install Quassel on Ubuntu/Kubuntu you have to activate the hardy-backports repository, as described in the guides for Ubuntu and Kubuntu.

Then just install quassel (or quassel-core if you only want to run the core component).

Ebuilds for Quassel are in Portage (net-irc/quassel). In addition to the current release, you will also find a live ebuild (quassel-9999) that will allow you to build the current version from Git (usual disclaimers apply). In order to use it, you'll need to keyword it by putting the following line into your /etc/portage/package.keywords:
net-irc/quassel **