Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Change
VEL_USE
Parameter: Direction
VEL_DIR
Parameter: Variation
VEL_DIRVAR
Parameter: Speed
VEL_SPEED
Parameter: Variation
VEL_SPEEDVAR
Parameter: Evaluation Type
VEL_TYPE
Parameter: Per Second
VEL_PERSEC
Parameter: On
IN_VEL_ON
Parameter: Animation Time
IN_VEL_ATIME