1. To display the Mean or Median, you need to have a Graph open with a numeric attribute (variable) on the desired axis.
2. First, as shown in the video, remove the attribute on the other axis by clicking the gear icon in the upper-right corner of the Graph and choosing Remove X Attribute or Remove Y Attribute.
3. Go back to the Graph Gear menu and select either Show Mean or Show Median..
4. Hover the cursor over the line that was created to display the value of the Mean or Median.