I do hope that you are not running as root, this is a really bad idea.
Is ns2 installed in /root. If it is, it will only be accessible to the root user, not regular users. Try installing it to /usr/local.
Once you have installed ns2 to /usr/local, try making a symbolic link to the ns2 executable in /usr/bin.
ln -s /usr/local/ns2/ns-allinone-2.28/bin/ns /usr/bin
Assuming that /usr/local/ns2/ns-allinone-2.28/bin/ns is the path to ns.
By making symbolic links, you remove the need to edit your $PATH.
You might need to make a link in a similar fashion to nam (I think)
I hope this helps