You will need to register at swisdev.oracle.com/DAX to get access to VMs which will contain the DAX APIs as well as sample code. You can look at the descriptions of the sample code in this article. You find the description of what was done with Apache Spark here.
There are two files uploaded in SWiSdev.oracle.com/DAX. One describes the low-level features covered by the DAX functionality and is offered as a shared library in Solaris, called libdax.so. The other describes "solver" functions that are built on top of this library and are written in terms of different data types and operations, rather than in terms of hardware capabilities.
A generic description of these is contained when you click on "DAX APIs" link in the dashboard you see when you login (see the "Resources" box to the right)
Both of these are changing, and we'll be upgrading them as we go along. Of course, your feedback is solicited; please use the Contact us or mailto: in the bottom of those pages to get back to us on these.