Go to system->Administration->Synaptic package manager. My system is not in english so it might be a little bit different.
Once you have the package manager open, go to configuration repositories. Select one that suits you and update sources. After that, try to search the package. If it still doesn't appear, you might need to add repositories manually to your /etc/apt/sources.list.
If you manually update sources.list you will have to apt-get update first, and then apt-install.