To restore a file or folder in subversion you must copy the older version of the file forward to the HEAD.

svn copy https://svn.example.com/trunk/foo/bar@24178 https://svn.example.com/trunk/foo/bar -m "Restoring deleted file"

where @24178 is the version number of the file/folder you wish to restore. This command operates directly on the repository so be careful!