I can mark a square with X or with a circle, but I still can't to mark with the + sign.
\usepackage{chessboard}
\begin{document}
\begin{center}
\newchessgame
\chessboard[maxfield=c4, color=black,
markstyle=cross,markfields=a2,
markstyle=cross,markfields=a3,
markstyle=cross,markfields=b1,
markstyle=cross,markfields=b2,
markstyle=cross,markfields=b3,
markstyle=cross,markfields=b4,
markstyle=cross,markfields=c2,
markstyle=cross,markfields=c3,
pgfstyle={[fill]circle},
markarea=\mycenter,
padding=-0.8ex,color=black,
markfields=a1,
markfields=a4,
markfields=c1,
markfields=c4, showmover=false]
\end{center}
\end{document}