Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Mass
MASS_MASSParameter: Variation
MASS_VARParameter: Over Age
MASS_OVERAGEParameter: Age Gradient
MASS_AGEGRADParameter: On
IN_MASS_ONParameter: Animation Time
IN_MASS_ATIME