Centering figure captions in IEEEtran

Adding

\usepackage{caption}% http://ctan.org/pkg/caption

to your document preamble (using the menus Document > Settings... > LaTeX Preamble) centres the captions by default. Although this also changes the default formatting, it would be possible to adapt it to match that of the default IEEEtran document class.

If this is for a journal submission, it is advised to stick to their formatting.

Just Put the following code after \documentclass[journal]{IEEEtran}

\makeatletter
\long\def\@makecaption#1#2{\ifx\@captype\@IEEEtablestring%
\footnotesize\begin{center}{\normalfont\footnotesize #1}\\
{\normalfont\footnotesize\scshape #2}\end{center}%
\@IEEEtablecaptionsepspace
\else
\@IEEEfigurecaptionsepspace
\setbox\@tempboxa\hbox{\normalfont\footnotesize {#1.}~~ #2}%
\ifdim \wd\@tempboxa >\hsize%
\setbox\@tempboxa\hbox{\normalfont\footnotesize {#1.}~~ }%
\parbox[t]{\hsize}{\normalfont\footnotesize \noindent\unhbox\@tempboxa#2}%
\else
\hbox to\hsize{\normalfont\footnotesize\hfil\box\@tempboxa\hfil}\fi\fi}
\makeatother

% will definitely work. 

This is the above code for JOURNAL class. Thanks to http://www.michaelshell.org/tex/ieeetran/