As far as I know, it's not possible to label the atoms in the 2D data display. It is, however, possible to select three atoms and display the plane which cuts through all of them, which might help you to figure out what you're looking at. To do this, select three atoms using the angle measurement tool. Next, go to the 2D data display, click 'Slice...' and then click 'Calculate the best plane for the selected atoms'.