The most recent stable version of Avidemux should be available in main Ubuntu apt-repositories. You may need to enable the universe and multiverse in your sources list before you can use search to find it. There are two methods of enabling those repositories for your system:

• For normal users, you can use the more user friendly program called Synaptic (for GNOME) or KPackageKit (for KDE) to edit your sources list, enable those repositories, and then save the changes.
• For advanced users, using either sudo or becoming root, you can manually edit the file /etc/apt/sources.list. You can uncomment the lines which refer to enabling the universe and multiverse.

After you have updated your sources list, you will need to grab the new package lists.

• For normal users, you can use Synaptic (for GNOME) or KPackageKit (for KDE) to update your lists.
• For advanced users, you can run sudo apt-get update.

Now you can search for the package called avidemux.

• For normal users, you can use Synaptic (for GNOME) or KPackageKit (for KDE) to search.

Once you have found right package, you can now install it.

• For normal users, you can use Synaptic (for GNOME) or KPackageKit (for KDE) to mark the package for installation and then proceed with the install.
• For advanced users, you can run sudo apt-get install avidemux.

After apt-get or Synaptic (for GNOME) or KPackageKit (for KDE) finishes, Avidemux should be installed on your system.

Nonetheless, you can also compile the program yourself from SVN if you want. See the article on compiling Avidemux.