Which version of Ubuntu? Is it using Unity or GNOME 2.x? I think that it is
GNOME 2 from the sounds of it, but am unsure. If so, you can open Nautilus,
e.g by clicking Places -> Home Folder. In the main window select the folder
you want to add and drag it into the Places menu on the left. Unity works
differently and there is no places per se, but there is a lens for that.
But its configurability depends on which version as well. It is easier to
add a bookmark in Nautilus by dragging and dropping a folder or location
from the right pane to the bookmark section on the left.