Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Layer Value
ID_MT_LAYER_VALUEParameter: Mode
ID_MT_LAYER_MODEParameter: Mute
ID_MT_LAYER_MUTEParameter: Solo
ID_MT_LAYER_SOLOParameter: Passthrough
ID_MT_LAYER_PASSTHROUGHParameter: Active
ID_MT_LAYER_ACTIVEParameter: Quaternion Interpolation
ID_MT_LAYER_QUATERNION