Prior to attempting to install the Nessus vulnerability scanner on Kali Linux, you will need to obtain a plugin feed activation code. This activation code is necessary to acquire the audit plugins used by Nessus to evaluate networked systems. If you are going to be using Nessus at home or exclusively within your lab, you can acquire a home feed key for free. Alternatively, if you are going to be using Nessus to audit production systems, you will need to acquire a professional feed key. In either case, you can acquire this activation code at http://www.tenable.com/products/nessus/nessus-plugins/obtain-an-activation-code.