Hi Jiyoon,
the most recent release you can get is ns-src-current_tar.tar, available
from http://mash.cs.berkeley.edu/ns/ by klicking on
"NS-2 experimental code (nightly snapshot of development tree)"
This is ns-2.0b18.
I'm not one of the developers, so if I'm wrong, somebody might let me (or
you) know.
-Chris.