After you have hit open , you should get this box pop up on your screen. This is the folder which your new worlds should be in after downloading it. Select & hi-light the world that u wish to open, I selected HansonStage as in the picture below. Hit the open button within the box.

Now you will see a Page with a yellow blob in it called HansonStage, As showing in the picture below. This is the icon that will start HansonStage up, or in your case, The world that u wish to open. i.e Rave world or Waterworld.

Double click on this icon. then you will be teleported to the world that u have selected.

After u have teleported to the world that you want. Mark the Start of the world , So that you don't have to go though this process again. hope this was enough information to get you into the world that you want. any comment's would be greatfully welcomed