Which linux distribution are you running on? The error you get is because a package with that name does not exist on your installation sources, that is the software repository configured on your system. Depending on the linux flavour you're running on, there are different procedures to enable additional repositories.
Furthermore on the command line you have apt-get on debian/ubuntu, yum on red-hat like systems and zypper on opensuse, just to cite some of them. A .rpm extension means it is a package managed by RPM (formerly red-hat package manager), the one you install using the rpm command or better using yum or zypper. On debian/ubuntu the packages have the .deb extension. On slackware you will have .tgz and so on. Just focus on the ones available for your system and you will be happy!