Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Formula
GV_FORMULA_STRING
Parameter: Use Port Names
GV_FORMULA_USE_PORTNAMES
Parameter: Angle Type
GV_FORMULA_ANGLE
Parameter: Value
GV_FORMULA_INPUT
Parameter: Output
GV_FORMULA_OUTPUT