It is not possible to apply a curve fit to a Histogram plot. You can get around this by using the Bin Data command from the Functions menu. Use Histogram as the Export Format. This will enable you to create a Column plot of the binned data and apply a curve fit.
KaleidaGraph is a thoughtfully designed graphing and data analysis application for research scientists, as well as for those in business and engineering fields. It produces publication-quality graphs, and easily converts the most complex data into a functional display.