Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Mass
MASS_MASS
Parameter: Variation
MASS_VAR
Parameter: Over Age
MASS_OVERAGE
Parameter: Age Gradient
MASS_AGEGRAD
Parameter: On
IN_MASS_ON
Parameter: Animation Time
IN_MASS_ATIME