Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Repulsion
RB_REPULSION
Parameter: Bounce
RB_BOUNCE
Parameter: Bounce Elasticity
RB_ELAST
Parameter: Mass Dependent
RB_MASS
Parameter: On
IN_RB_ON
Parameter: Animation Time
IN_RB_ATIME