This is by far the easiest question you have asked so far. This is one of the basic operating modes of ADS. The custom symbol, when it is defined, is used whenever you the design itself is used in any design heirarchy. There are three easy ways to re-use any design but unfortunately one of them is not the palette.
Either of the following methods will work:
1. Open both the ADS main window and the schematic window where you wish to re-use design together. In the main window find the ATF54143_dt design in the "Project View" list. Now simply click and drag the design into the schematic window (keeping the mouse button held down). Click on the required location in the schematic window to place the symbol.
2. In the schematic window open the Component Library List. It is a button on the second toolbar that looks like a set of books or the menu command Insert > Component > Component Library... In the resulting window select the Projects line in the left panel. Again find the ATF54143_dt component in the right panel and click and drag it to the schematic window.
3. In the schematic window in the Component History toolbar dropdown list, the second one on the second toolbar next to the Palette selector list, simply type in the name ATF54143_dt and press enter. Again click the location in the schematic window where you wish to place the symbol.
Wire up the rmainder of the design and all is done.