How can I use Source Sans Pro in Tex-Live 2012?

You should get an updated version of TexLive - which you obviously did not do before. The latest version includes Source Sans Pro.

It is however not in the official repository for 12.04 and 12.10, so you have to add an unofficial one:

$ sudo add-apt-repository ppa:texlive-backports/ppa

After that, you can install TexLive with apt, not forgetting to get the font package:

$ sudo apt-get install texlive
$ sudo apt-get install texlive-fonts-recommended texlive-fonts-extra

You can now use Source Sans Pro:

% LaTeX
\usepackage[default]{sourcesanspro}

Or for XeLaTeX:

% !TEX TS-program = XeLaTeX
\setsansfont{Source Sans Pro}