You might want to look at the example file 5ZoneAirCooledWithSlab.idf which integrates Slab as part of the preprocessors so that all the input is in one file. The objects GROUNDHEATTRANSFER:SLAB:* are part of a normal IDF file.
In addition, unless you need something specific that is provided in Slab, you might want to consider a newer model that has been added to EnergyPlus called Kiva. See the Foundation:Kiva description in the InputOutputReference and in the ZoneCoupledKivaBasement.idf example file. There is also the Site:GroundDomain:Basement object.