If a package is not in the Ubuntu repositories, you may need to install it by downloading the Deb package file. A Deb file can usually be installed by opening it using the Ubuntu Software Center. But this does not work properly sometimes.
If you prefer to use a GUI program, install the package gdebi. After installation, you will be able to install Deb files by right-clicking it in Nautilus and choosing Open With.
Tried with: GDebi 0.8.5 and Ubuntu 12.04.1 LTS