And why you think this is a problem ? The first time you installed, it ASKED your for a destination. The 2nd installer, is not allowing you change the choice because, even if know you have the FREEDOM to install anywhere, all the sceneries must still be together.
That's what the installer dialog says anyway:
If you already installed another product which uses the Addon Manager this installation will use the previous folder automatically.
If you think you made a mistake by selecting the original folder, you must Uninstall everything, and reply YES to the question "Do you want to remove the Addon Manager?", so the next time, you'll be offer again a choice.