It looks like the browsing has some issues in the latest launcher version, our devs are looking into that to fix this in the next Launcher version, thanks for reporting the issue ! There's actually a workaround in order to set the path to your Unity.app: just type it manually in the " browse for executable " field. Your path should in theory be /Applications/Unity/Unity.app
From there the launcher should recognize your .app and your Unity version so that you can carry on with the installation.
Hope this helps !