You need to first draw a track around the area you wish to measure. Do that using the draw / pencil tool. The only trick here is that you need to right click somewhere (anywhere) on the map when you are done drawing, then you'll get a dialog that let's you choose "save as...". Save the file out, then switch back to the regular hand/pan tool mouse mode. You should now be able to right click anywhere on your new track and get a dialog that lets you choose "compute area."
Hope that helps!