Welcome to LQ.
# yum install \
libX11-devel libXext-devel libXau-devel libXmu-devel \
gcc-c++ libtool autoconf automake
tar xvf ns-allinone-2.34.tar.gz :
Then you have /home/<name>/ns-allinone-2.34/
cd ns-allinone-2.34/ && ./install
..
Other info :
LQ Search .. keyword = ns2
..