What do you mean 'not distributed on the package manager'? Nothing is distributed 'on' the package manager. The package manager is APT. The APT system downloads packages, installs packages, updates packages, and resolves dependencies. The packages it works with are already binary packages parceled up in the .deb format. It gets the packages from an online repository, or repo- in our case the Trisquel repo. This is the server where all the packages are stored for downloading (such as archive.trisquel.info, or any of its numerous mirrors).
Simply issuing the apt-get install icecat command will retrieve and set up IceCat for you.
