Computing average and other statistics, such as STD, maximum and minimum, can be done in a spreadsheet by exporting the data, but it can also be done directly in the plot. Doing it directly in the plot is faster.
To do this, first plot the curves you want to compute the mean value from. Make sure the button is on so the curves are plotted on top of each other. The button should also be turned on to display the legend list shown to the right in the figure below.
Now use Plot->Add Curve(s)... to open the dialog shown below
Select the radio button to plot statistics.
Then mark the curves you want to include for computing the mean value. In the example above, all three curves are marked initially. Then click the button to add the curve and to exit the dialog. The new curve is named Mean and is shown in the figure below