Skype is not available in any Ubuntu software repository. So it can't be directly installed using Synaptic or apt-get. To install it using Synaptic or apt-get we have to add Medibuntu repository. But I'm too lazy to install it in this way. I rather download Skype from Skype download page. Skppe can also be downloaded for Ubuntu 64 bit. Then install it by double-clicking the deb file and clicking "Install package".