\documentclass{article}
\usepackage{graphics,graphicx}
\usepackage{pstricks,pst-node,pst-tree}
\begin{document}
\pstree[levelsep=35pt]{\Tcircle{10}}
{
\Tcircle{11} \Tcircle{21}
}
\end{document}
Why the error:
! Undefined control sequence. \c@lor@to@ps l.5 \pstree[levelsep=35pt]{\Tcircle{10}} ?
I compile with latexmk if that makes any difference:

\listfilesin front of the code and show the output of this command that you easily find in the.logfile. – egreg May 25 '11 at 10:16pdflatex, which does not work withpstricks. Try usinglatexmk -pdfdviin place oflatexmk. – Ian Thompson May 25 '11 at 14:48dvipdfby default, and I think that uses ghostscript anddvips. It works fine on my system, but maybe-pdfpsis a safer option. – Ian Thompson May 25 '11 at 15:02