When you press on the "+" at the top right of the App Launcher screen, a new window goes up, right? Scroll this window until you see the app you want, and then press the Add button. Most Macs or PC have many dozens of apps, so they all cannot be presented at once. The apps are presented in alphabetical order.
Alternatively, you can type the name of the app you are looking for in the search field at the top of this window to find the app you want to add.