[pstricks] This is embarrassing

[Michael Gallagher]

  I have to be missing something!!

I am trying to do some very simple flow diagrams for a math text -- 

This one generates an "undefined control sequence" right after the "mnode=circle" parameter.  When I remove that, no errors. 

I have been checking around last couple days on what seems to be a simple problem, a command that is used everywhere --

This code is as near as I can tell exactly what M. Joshi shows in his paper, "Create Trees and Figures in Graph theory with PSTricks," on the Internet.

Can someone tell me what is wrong here??

Thanks and best regards

Mike Gallagher

Code follows

\documentclass{article}

\usepackage{graphics,graphicx}
\usepackage{pstricks,pst-node,pst-tree,pstricks-add}

\begin{document}

$
\psmatrix[colsep=3.5cm,mnode=circle]
M & U \\
\endpsmatrix
$



\end{document}