Here's a example I made that hopefully is what you want.
What I've done there is this:
I create the background, the player and a invisible sprite that I called "camera".
"camera" has the scrollTo behaviour, so the camera is always following this sprite.
I align the camera sprite in the middle of the window, by clicking in it, "Align" -> "Window" -> "center horizontal" / "center vertical".
Then in the event sheet I see if the player passes the viewportRight or Left. If yes, I move the "camera" sprite according to the windows width size of the project. In this case it's 640, so I put 640. If you ever change the windows width in the Project Properties, change the 640 to the new value you changed. (but don't forget to align horizontal/vertically again, otherwise it will not work as expected).
I hope this is what you want and that my explanation is clear. English is not my first language.
There's an error if you go all the way to the end of the layout and then go back. The camera don't align properly... well, I don't now how to fix it. I'd try to take a look at it now, but new it's christmas and I don't have that much time. Sorry.