Install the AUR package.
Register your email at http://www.tenable.com/products/nessus/nessus-plugins/obtain-an-activation-code and wait for your key to be emailed to you.
nessusd.service unit file, see systemd#Using units for details.
Access the web interface at https://localhost:8834 and/or use the commandline interface (
/opt/nessus/sbin/nessuscli). In most browsers, you will need to manually accept the SSL certificate you created for the server.
The package can be removed with pacman, but files created by Nessus, such as the plugin database it downloads, must be removed manually:
# rm -r /opt/nessus