There are probably better ways to do this, but since you only wanted the mean or the median, here is an interactive, but automated approach.
First, create your boxplots as you have done. Use the shift key and select the bar chart icon along the top. This should put the bars on top of your boxplots. Change the bar style to "Float". Change the statistic to median or mean, your choice. For the label field, choose Label by Value.
This approach will emphasize your mean or median a bit more and places the label next to where it occurs. Further, if you want the mean, the mean is not shown on the boxplot, so this puts the mean on the boxplot.
Dan Obermiller