Du musst einfach nur `\pgfmathprintnumber` verwenden anstatt `\pgfplotsretval` direkt zu expandieren (Das ist das, was `pgfplotstable` intern auch macht).
\begin{filecontents}{n5.txt}
5 0 0.59049 0.32768 0.23730 0.16807 0.07776 0.03125
5 1 0.91854 0.73728 0.63281 0.52822 0.33696 0.18750
5 2 0.99144 0.94208 0.89648 0.83692 0.68256 0.50000
5 3 0.99954 0.99328 0.98438 0.96922 0.91296 0.81250
5 4 0.99999 0.99968 0.99902 0.99757 0.98976 0.96875
\end{filecontents}
\documentclass{article}
\usepackage{pgfplotstable}
\def\getcell#1#2{%
{%
\pgfplotstablegetelem{#1}{#2}\of{\nV}%
\pgfmathprintnumber{\pgfplotsretval}%
}%
}
\begin{document}
\pgfplotstableread{n5.txt}{\nV}
\pgfplotstabletypeset{\nV}
$P(X \leq k) = F(n,p;k) = \getcell{0}{2}$
$P(X = k) = P(X \leq k) - P(X \leq k-1) = F(n,p;k) - F(n,p;k-1) = ???$
\end{document}
![alt text][1]
[1]: http://texwelt.de/wissen/upfiles/test_152.png
