Check the right most side of the toolbar. Are there two little down arrows? If so, it means there are more icons than can be displayed at once on your toolbar. Click on that and the rest of the icons will appear.
If you don't have those little down arrows then you need to add the FFDL icon to your toolbar.
Preferences=>Toolbars=>The Main Calibre toolbar
You then need to find the FFDL icon and click the arrow to add it to your toolbar
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