Hi,
The new Windows 8.1 "Metro" User Interface uses different concepts than the "traditional" desktop. It no longer pins newly installed apps on your Start screen.
As suggested by Don, the easiest way to find an application that you've installed is to invoke the Windows 8.1 search function.
You can also find an application by checking "All Apps" tile view, the "All Apps" view lists all of the installed programs on your computer. Just right click anywhere on the Start screen to display a pop up task bar at the bottom of the
screen. On the far right end of this task bar, click "All Apps" icon.
After you find the application, you can right click the tile and choose to "Pin Tiles to the Task Bar", this way you don't need to search every time to find your application.
Regards,
Ethan Hua
TechNet Community Support