OSX

Launch the Terminal from Utilities and type the following command:

• sudo launchctl load -w /Library/LaunchDaemons/com.sidefx.sesinetd.plist

You will be prompted for a password. This is your password for the machine.
This will clear the disabled flag for the license server daemon.

Fedora/RedHat/CentOS

The culprit is most likely SELinux. Please see this faq entry for how to set up an SELinux policy.

Other

Check that your operating system meets our system requirements