Tcl wasn't installed properly

[bhargavigoswami at gmail.com]

I faced the same problem during my last installation of ns2.35 in ubuntu 11.04.
After I install ns2.35, got message of successful installation of ns. Then I set path in /.bashrc. Then I gave ns command which gave me same error which you got. 

The problem is because, ns executable is also at /usr which is conflicting.

Solution: 
1. Go to location root-usr-local-bin by giving following command in terminal
cd /usr/local/bin
2. There you would find the ns file. We just need to remove it by giving following command
rm ns
3. Thats it, you are done. Now your ns starts running successfully.

Happy Learning.....