Hi Thomas, I'm talking about Pro as well.
1. In the Contents pane in a layout view, expand the legend so you can see the items within the legend
2. Select a legend item inside the legend in the Contents pane
3. Right-click on the legend item in the Contents pane and choose Properties
4. In the Properties pane, at the top, it should say Element; below that should be the legend name, colon, legend item name; below that it should say Properties | Text Symbol. Make sure Properties is selected; directly below that it should say Show with the checkboxes below that.