Basically it means your PGF installation is waaay old. Like from 2007 if you run Ubuntu 8.04 as I happen to. If you want to find out what your version is compile this short tex file:

\listfiles 
\documentclass[]{article} 
\usepackage{tikz} 
%\usetikzlibrary{positioning} 
\begin{document} 
a 
\end{document}

(found in http://old.nabble.com/I-can’t-find-file-`pgflibrarytikzpositioning.code.tex’-td20634766.html)

After that grab a fresh copy from http://sourceforge.net/projects/pgf/. As root remove (or back up somewhere else) /usr/share/texmf/tex/context/pgf , /usr/share/texmf/tex/generic/pgf , /usr/share/texmf/tex/latex/pgf and /usr/share/texmf/tex/plain/pgf. Extract the new pgf and copy the corresponding folders to the same places, run (as root) mktexlsr and you are done.