from: http://wiki.ubuntu.org.tw/index.php/HowtoInstallNS2

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$ vi /etc/apt/sources.list
# ¼Óµ½×îÏÂÃæ
deb      http://bode.cs.uni-magdeburg.de/~aherms/debian sid ns2
deb-src http://bode.cs.uni-magdeburg.de/~aherms/debian sid ns2
$ apt-get update
$ apt-get install ns

For more, you can visit the web link above.