[pstricks] How to get this to work?

[David Arnold]

All,

I've got:

\begin{equation*}
\frac{#a}{#b}=\frac{#c}{n},
\end{equation*}
cross multiply.
\begin{equation*}
\psset{linecolor=red,nodesep=-15pt}
\frac{\rnode{A}{a}}{\rnode{B}{b}}=\frac{\rnode{C}{c}}{\rnode{D}{d}}
\ncline[arrows=->]{A}{D}\uput{20pt}[-20](D){\color{darkred} 
\text{Product of extremes}=ad}
\ncline[arrows=->]{B}{C}\uput{20pt}[20](C){\color{darkred} 
\text{Product of means}=bc}
\end{equation*}

We're compiling the document using a script. Here are the pertinent  
lines:

     system "latex --shell-escape $testbank-preview.tex";
     system "dvips -o $testbank-preview-pics.ps $testbank-preview.dvi";
     system "ps2pdf -dAutoRotatePages=/None $testbank-preview-pics.ps  
$testbank-preview-pics.pdf";
     system "pdflatex $testbank-preview.tex";

How can I get the arrows in the above code snippet to show properly  
using our compilation method?

D.