I am looking at your project, and I am going to have to make some guesses as to what I think is going on here.
When you say, “adding more sections those are added below the header and not as the “next” section below at the bottom,” I BELIEVE what is happening is that your order of elements is just mixed up. You definitely do not want to get into using absolute positioning for everything (and you don’t need to).
Check your Navigator (hit F) and make sure that your new section is at the bottom. Right now you’ve got a div called “div section” with relative positioning. In reality, that could just be a section, there is no need to use a div there.
Also, for the record, here are the settings for your MacbookContainer element. Take note of the background position settings. This is important.
If you’re stuck trying to figure out how to get a new section to appear as the “next” section on the page, just select the body itself (by clicking on the word “body” at the bottom of the screen), then create a section. That will automatically add it to the very end of the current document.