If you published a tree from Family Tree Builder that had sources, then you will see a new Sources tab under Family Tree > Sources. That is where you will find your already existing sources, and where you can add new ones.
You can add and sort the sources according to family trees on your site.
If you do not yet have any sources on your site, then go to the profile page of an individual in the tree to add them. Choose to edit the profile. Select source citations in order to add a new source.