Hi.
As you told me I tyoed:
soheil@soheil-desktop:/media/TOSHIBA$ sudo nano /etc/apt/sources.list
[sudo] password for soheil:
soheil@soheil-desktop:/media/TOSHIBA$ apt-get install mpich
But got:
E: Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
soheil@soheil-desktop:/media/TOSHIBA$ sudo apt-get install mpich
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mpich is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'mpich' has no installation candidate
soheil@soheil-desktop:/media/TOSHIBA$