I followed this instruction http://orgmode.org/worg/org-contrib/babel/languages.html#configure

and added the following to my .emacs file. 

After that, org-mode mysteriously switched back to the old version 7.9.  Another problem is that export command C-c C-e no longer works and generates error: Symbol's function definition is void: org-defvaralias. Is there any step I missed? Thanks. 

Shiyuan