Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Repulsion
RB_REPULSIONParameter: Bounce
RB_BOUNCEParameter: Bounce Elasticity
RB_ELASTParameter: Mass Dependent
RB_MASSParameter: On
IN_RB_ONParameter: Animation Time
IN_RB_ATIME