PSTree creates confusing Undefined control sequence

Question

This bit of code generates an undefined control sequence on the last curly brace. What am I doing wrong?

\psset{levelsep=1cm,treesep=.75cm,nodesep=4pt}
\begin{center}

\pstree{\Tr{$z$}}
{
    \pstree{\Tr{$y$}}
    {
        \pstree{\Tr{$w$}}
        {
            \Ttri{A}
            \pstree{\Tr{$x$}} 
            {
                \Ttri{B}
                \Ttri{C}
            }
        }
        \Ttri{D}
    }
    \Ttri{E}
}

\hspace*{0.5in}
into
\hspace*{0.5in}

\pstree{\Tr{$x$}}
{
    \pstree{\Tr{$w$}}%
        {
        \Ttri{A}
        \Ttri{B}
    }
    \pstree{\Tr{$y$}}
        {
            \Ttri{C}
            \pstree{\Tr{$z$}}
            {
                \Ttri{D}
                \Ttri{E}
            }
        }
}

\end{center}

Answer 1

Run the file with xetex or load the package auto-pst-pdf and then run it with pdflatex -shell-escape <file>. See also http://tug.org/PSTricks/main.cgi?file=pdf/pdfoutput

Answer 2

\usepackage{pst-tree}

If there is still errors, please upload the full (minimal) source file and log file.