Measuring tool is a web only feature
To measure an area:
Open the measure tool on the right side of the map
Draw your shape on the map. This can be a line, areas or circle or area. Select what you would like to draw by clicking on it and then draw it by clicking on the map to add points, finish your shape if it is a line or area by double clicking.
The measurement will be shown in the measure tool box
After drawing your area / line or circle you can choose to discard or save as a field, feature or note.