Family Tree Builder trees are currently not editable from your family site. Family Tree Builder trees are Published to a family site so they can be viewed by your family members, but all editing occurs in Family Tree Builder and not on your family site.
If you would like to have a family tree you can edit online, do the following:
Open your family tree in Family Tree Builder.
Go to File > Export GEDCOM.
Now go to your family site.
Click on Family Tree > Import tree (GEDCOM).
Browse your computer to find the GEDCOM you just created.
Click upload and that’s it!
Now you have a family tree that you can edit online.