To make apt-get or “Add applications” show more software, enable more software repostories.

You can make the change in either command line or the graphical user interface, no need to do both.

Sometimes you need a piece of software that’s not available in the default repositories (main and restricted). For example, apt-cache shows 0 hits for your program:

 $ apt-cache search system-config-kickstart
 $

You can add more repositories with

 $ sudo nano /etc/apt/sources.list

deb     http://fi.archive.ubuntu.com/ubuntu/    breezy          main restricted universe multiverse
deb-src http://fi.archive.ubuntu.com/ubuntu/    breezy          main restricted universe multiverse
deb     http://security.ubuntu.com/ubuntu       breezy-security main restricted universe multiverse
deb-src http://security.ubuntu.com/ubuntu       breezy-security main restricted universe multiverse
deb     http://archive.ubuntu.com/ubuntu/       breezy-updates  main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/       breezy-updates  main restricted universe multiverse

$ sudo apt-get update

Now apt-get can install a lot more packages, for example:

$ apt-cache search system-config-kickstart
system-config-kickstart - graphical tool for creating Kickstart files

See: The Ubuntu Starter Guide: How do I add Universe and Multiverse?