Double click on axis to open axis settings (or use right click)
Add lines you need and press OK
Using JMP > JMP Reports > Change the Appearance of Graphs > Customize Axes and Axis Labels in Graphs - Reference Lines
To add to that, if you want the reference line to be along the count axis, from the red popup choose Histogram Options > Count Axis. Then double-click the count axis as @jthi describes.
