may package list is missing

Dear friends:
I installed the debian with a usb disk with kde selected, and i disable the source from net during the installation process.
I delete the lines "cdrom" from the source.list file, and add new source to it.
But now, the kde software mangement only show the package installed.It seems the kde software mangement did not refresh the package list from the new software sources.
I tried the command :
apt-get install gnuplot,
It gives me the following feedback:

Package gnuplot is not available.
Could you please give me some suggestions.
Regards

Did you run apt-get update after editing your sources.list? You need to run it in order to update the list of packages available in the repositories.

There are several questions raised by your post, all of which should have been addressed by someone with familiarity with this, or any other Gnu/Linux, forum which from your join date you should have.

What version of Debian are you running?


What sources did you, exactly, add to the sources.list?

According to apt-cache, here in Debian Sdi, this is available in the Debian /main repo.

If you are using Debian stable the only entry you should need in the sources.list to get this package is;

deb ftp://ftp.us.debian.org/debian/ wheezy main

The entire official Debian Stable sources follow. The source code entries are commented out.

deb ftp://ftp.us.debian.org/debian/ wheezy main contrib non-free
## deb-src http://mirror.steadfast.net/debian/ wheezy main contrib non-free

deb http://security.debian.org/ wheezy/updates main contrib non-free
## deb-src http://security.debian.org/ wheezy/updates main contrib non-free

## wheezy-updates, previously known as 'volatile'
deb ftp://ftp.us.debian.org/debian/ wheezy-updates main contrib non-free
## deb-src http://mirror.cpsc.ucalgary.ca/mirror/debian.org/debian/ squeeze-updates main

If you have need or, or want packages from the deb-multimedia repo (3rd party, unofficial repo for people using Debian);

## debian multimedia
deb http://www.deb-multimedia.org wheezy main non-free
## deb-src http://www.deb-multimedia.org wheezy main non-free

If you are not using Stable you will need to edit all instances of "wheezy" to reflect your version. If you are using Sid (unstable) remember tha only the first line is used, others will return errors.

deb-multimedia does maintaing a sid repo.