First to define the system. Concrete core heating/cooling (often also called "concrete core activation"), is where warm/cool air is passed through holes within concrete slabs to heat the building fabric directly, before supplying the air into the occupied spaces.

This system can be modelled in DesignBuilder by using a Detailed HVAC supply air plenum set up in such as way that a zone is effectively used to represent the holes in the concrete. The HVAC plenum connection allows air from the AHU to be passed through the plenum zone before being delivered to the occupied spaces. 

So for example to model a 300mm thick concrete slab with air ducts in the middle, a "building zone" would be defined between stories, much like a standard plenum zone and the ceiling and floor of the plenum zone is set up using 150mm concrete constructions. It is important to set correct values for the convective heat transfer coefficients in the core zone and you may need to experiment with different values to obtain the effect you need.