My computer couldn't find the package install for a flash plugin for firefox. I fixed the problem by showing packages from all repositories and then downloaded the plugin.
You want to run the command
where pkgname is the name of a package. If you get an error of the type - E:Couldn't find package pkgname - then you don't have package pkgname in the system.
You can search for the package in your system using the full name or part of the name:
This returns the list of packages that contain pkgname in the package name or description.
The reason for the package not being present could be because the repository that contains the package is not included in your system. In such cases, you can follow the Ubuntu Community Documentation on including repositories
Also you can use synaptic : SynapticHowto
it is easier than command in terminal .
If you know from various forums that the software you are looking for is in the ubuntu universe or related repositories then....
It may be as simple as go to system> administration> software sources
enter your account password which may be same as super user (you can also set super user or "su" password like 'sudo passwd root' for future if you haven't already)
go to "third party" tab
and check all items!
close and see if that works for installation of any packages like "sudo apt-get install blah blah blah"!