Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Slice
PRIM_SLICEParameter: From
PRIM_SLICEFROMParameter: To
PRIM_SLICETOParameter: Regular Grid
PRIM_REGULARParameter: Width
PRIM_REGULARWIDTH