First off, the concepts behind this model are credited to this:

http://www.spectrum-soft.com/news/spring2002/diac.shtm

Download the Diac.lib file to your sub folder.
Download the Diac.sym file to your sym folder.

Two circuits are available, one shows the Diac model netlist in schematic form.
The other shows a simple test of the model.

You must define the breakover voltage {Vk} and Series resistance {Rs}.
Also, you can play around with the "Vh" (hysteresis) value of the SW model to your liking.
If the model is missing anything or can be improved, please feel free!!