FWIW - the Nuitrack SDK is a bit of a minefield - just so you are aware.
If you use any part of the Nuitrack SDK in any application that you develop - each end user of YOUR application MUST purchase a separate individual licence of Nuitrack Pro as well and install that licence on their computer alongside your application.
The licence you purchase when you start developing with Nuitrack is for YOUR computer system only - and you cannot share or distribute that licence to your clients.
So yes you can download the Nuitrack SDK itself for free ... BUT you need to pay $59 for a perpetual licence that lets you RUN the Nuitrack Pro engine on your own computer - otherwise your code will stop tracking after 3 minutes.
Then when you sell your application that includes any parts of the Nuitrack SDK, your end user must ALSO pay $59 for a perpetual licence of Nuitrack Pro otherwise your application will revert to trial mode and stop tracking after 3 minutes.
Further the licence is per PC as well - it is handled by a separate registration application that binds to an individual PC.
So for example it you have multiple machines for development and test - each machine will need its own Nuitrack Pro licence.
And yes the documentation is pretty bad and far from user friendly - which is ironic for a company trying to build a user pays model.