How can I install NST in Ubuntu? I have searched but found nothing.
The Network Security Toolkit is a Linux-based Live CD distribution(similar to Fedora/RedHat) and is not meant to be installed as discrete Ubuntu package.
There are numerous packages used in NST(i.e wireshark, ntop, and nmap) that can also be installed independently of the NST Live CD. But, you will need to find them separately and install them one-by-one through either Synaptic, the command line, or the Ubuntu Software center.
The NST ISO can be downloaded from Sourceforge.
For an overview of the NST package, see NST General Information.