First time here? Check out the Help page!
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.