The Launcher can only recognize binary versions of the engine that it downloads/installs. To open the source engine you can navigate to where it was installed and go into the Engine/Binaries/Win64 folders and open the UE4.exe. However, when you are using an engine built from source, the project associated with the source engine will show in the Library tab of the Launcher with other projects. Tim 's answer in the following post shows how to switch an existing project to run using the new source built engine: