On the "Generate new world" dialog, check the box to use a custom heightmap, then click the 'select' button. It will open a file selection dialog. If you haven't added any heightmaps, then the only one should be the Earth heightmap: "CompositeEarth_3600x1800.png". Click "Load" to close the dialog. The Earth map should appear in the generate world dialog. You can now click "Generate" to generate a new world using the Earth map. Note that the import process is not perfect and you will need to edit the map to make it look like the actual Earth.