You don't need to install Auto-Fit. If you've activated DAZ Studio 4 Pro as the Pro Edition, then you already have Auto-Fit. To check this, go to HELP --> About DAZ Studio 4. If it says 'Pro,' you have Auto-Fit. There is not a 'tab' for Auto-Fit. It appears if you have Genesis loaded and selected, and you choose to load clothing or hair for a fourth generation figure.
Using Auto-Fit is fairly easy. Auto-Fit by default will convert fourth generation clothing for use on Genesis. This is how to do it. Have the Genesis loaded and selected in the 'Scene Pane.' If it is highlighted in the Scene Pane (VIEW --> Panes --> Scene), then you know that it is selected. Next, double-click on the icon that will load the fourth generation clothing product. DAZ Studio 4 Pro will prompt you with the Auto-Fit option screen. You will see several types of add-ons. Choose the one that most closely matches the type of clothing you are trying to load. Then press 'Accept' and you will see Auto-Fit convert the rigging so that it will work on Genesis.
Auto-Fit no longer strips out the adjustment morphs in the clothes. These are preserved.