I Want to Install an Application That Is Not In Synaptic

Synaptic Package Manager 0.61 on ubuntuAlthough Synaptic contains a huge selection of packages, sometimes the package you need is not included. The first thing you should check is that you have enabled the additional repositories such as universe and multiverse. Open Synaptic from System > Administration > Synaptic Package Manager. Now click Settings > Repositories > Add. Ensure that all of the Components check-boxes are selected. See https://wiki.Ubuntu.com/AddingRepositoriesHowto for more details.

The Repository Run-Down

The universe repository contains the thousands of packages that are part of the Debian distribution, upon which Ubuntu is based. All of these packages are entirely free and supported by a community of Ubuntu contributors.
The multiverse repository contains a number of packages that are freely available to download but are not fully Open Source. If you only want to run Open Source software, you may not want to use this repository.

If you have enabled these extra repositories and your package is still not there, have a quick hunt around with a search engine to see if you can find a repository (known as a Debian or APT repository) for your package. If you find one, use the Repositories dialog box you have just played with to add the new repository, and then use Synaptic to install the package.

If no repository is available, look for a Debian package (.deb) for the application. If you find one, download it, and double-click it to install. If no Debian Package exists, look for an Autopackage. (Details about Autopackage installation are in the next section.)

Finally, if all else fails, download the source code and compile it.