Familysearch.org does not always have easy navigation. It may look pretty but there is so much available there that it is sometimes hard to find what you are looking for. Thank goodness for Google!
I could not find where to add my GEDCOM to their one world tree from the menus, so I googled around and found this URL for their user guide to uploading a GEDCOM which explains the details (no longer online)
As to why you want to do this, it is to use the cool tools at puzzilla.org (a blog post is coming on that soon)
Read on for my quick step guide to adding your GEDCOM at familysearch: