Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Change
VEL_USEParameter: Direction
VEL_DIRParameter: Variation
VEL_DIRVARParameter: Speed
VEL_SPEEDParameter: Variation
VEL_SPEEDVARParameter: Evaluation Type
VEL_TYPEParameter: Per Second
VEL_PERSECParameter: On
IN_VEL_ONParameter: Animation Time
IN_VEL_ATIME