Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Formula
GV_FORMULA_STRINGParameter: Use Port Names
GV_FORMULA_USE_PORTNAMESParameter: Angle Type
GV_FORMULA_ANGLEParameter: Value
GV_FORMULA_INPUTParameter: Output
GV_FORMULA_OUTPUT