To install the tuxguitar ubuntu package, before you must have installed that packages in your system:
to install that dependencies, you need have enabled the Universe and Multiverse repository.
Ubuntu as default, comes with a software called GDebi Package Installer, what can install debian packages in graphical mode. This software can also automatically install the package dependencies.
To install from a system terminal, first you need install the package dependencies:
sudo aptitude install sun-java5-jre libswt3.2-gtk-jni libswt3.2-gtk-java
Now all is ready, so we can go to the package downloaded folder and use dpkg to install it.
sudo dpkg -i tuxguitar-1.0-rc4-ubuntu-x86.deb