Ich weiß nicht, wie das am Schluß aussehen soll.
Vielleicht so:
![alt text][1]
\documentclass[margin=3pt]{standalone}
%\documentclass[fleqn,12pt,a4paper,german,xcolor=dvipsnames]{article}
\usepackage[ngerman]{babel}
\usepackage{diagbox}
\usepackage{longtable}
\usepackage{xcolor}
\usepackage{tikz}
\newcommand{\Marker}{%
\begin{tikzpicture}[inner sep=0pt, outer sep=0pt, remember picture]
\node[shape=rectangle](Marker1){};
\end{tikzpicture}
}%
\begin{document}
\begin{minipage}{0.5\textwidth}
\begin{longtable}{|l|}
\hline
\diagbox{\colorbox{gray}{\textcolor{black}{0}\textcolor{white}{0}}}{256}\quad\vline\qquad3345\Marker\\ \hline
\end{longtable}
\end{minipage}
%
\begin{tikzpicture}[remember picture, baseline = 1.75mm]
\node[draw, minimum height=1cm, rounded corners, fill=lightgray](Marker2){Das ist super...};
\end{tikzpicture}
\begin{tikzpicture}[remember picture, baseline = 1.75mm, remember picture, overlay]
\draw[<-, ultra thick, shorten >=2pt] (Marker1) -- (Marker2) node[midway, above]{Text};
\end{tikzpicture}
\end{document}
[1]: http://texwelt.de/wissen/upfiles/Namenlos-11e4yyy_5.png
