Open terminal.
type su
hit enter
then your root password
now you are root with # prompt for the session or until you enter 'exit'
And you should just use synaptic.
Or to explain it a bit more in depth:
The command
su stands for "substitute user" (not "superuser", as some people seem to think). It will let you run commands as
any other user on your system.
The basic syntax of
su is "su <name-of-substitute-user>". If you just type "su" and hit Enter -- without specifying which user you want to become, the system will take for granted that you want to become root.
But if "su" is followed by something else, the system will believe
that is the substitute user you temporarily want to become. So if you enter a line like
su apt-get remove floola you are not telling the system to run the program apt-get as root; you are telling it that you want to
become the user
apt-get -- which naturally doesn't exist.
Now, if you want a command that works almost like
sudo, try
kdesu. (You'll be asked for
root's password though.)