[pstricks] \naput does not work

[serpa at fem.unicamp.br]

Dear Sirs,

I am a new user of PsTricks. I am studying it with the book "The 
Latex Graphics Companion" by M. Goossens, S. Rahtz and F. 
Mittelbach, 1997.

My problem is how to write over connectors. 

Consider the code as an example: 

\begin{figure}[htbp]
\centering
\psset{arrows=->,mnode=circle}
\begin{psmatrix}
A & B \\
C & D
\end{psmatrix}
\ncline{1,1}{1,2}
\ncline{1,2}{2,2}
\ncline{2,2}{1,1}\naput{test}
\end{figure}

This code does not write the word test over the arrow that goes 
from position 2,2 to 1,1 as expected, i.e., \naput does not 
work. 

Can you give me an idea of how to solve this problem?

Note: 
The first lines of the document are:
\documentclass[a4paper,12pt]{article}
\usepackage[brazil]{babel}
\usepackage[latin1]{inputenc}
\usepackage{pstricks,pst-node} 

Thank you in advance.
--
Prof. Dr. Alberto Luiz Serpa
Department of Computational Mechanics
Faculty of Mechanical Engineering 
State University of Campinas - UNICAMP
e-mail: serpa at fem.unicamp.br
phone: (55)19-37883387 / (55)19-37883386
www.fem.unicamp.br/~serpa