DnD may be available in the future, since it was enabled once in the past.

In the meantime, just use the Library's File menu and choose "Add Folder to Share" to select new folders to add to the Library.

If you just want to add single files, drag them from the Finder to one of your already Shared folders. LW will see them the next time LimeWire is restarted, or the next time you hit the Refresh botton for that folder.