To calculate the intersection of two planes, add the two planes to the stereonet using the Add Plane option. The intersection of the two great circles gives you the intersection point. To get the coordinates, hover the mouse over the intersection point, and make sure that the Convention option for coordinates (click on the box in the lower right corner of the stereonet) is set to Trend/Plunge, so that the coordinates of the intersection line are displayed in Trend/Plunge format.