Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Enabled
ID_FIELDLAYER_ENABLEDParameter: Value
ID_FIELDLAYER_ENABLE_VALUEParameter: Color
ID_FIELDLAYER_ENABLE_COLORParameter: Direction
ID_FIELDLAYER_ENABLE_DIRECTIONParameter: Rotation
ID_FIELDLAYER_ENABLE_ROTATIONParameter: Layer Opacity
ID_FIELDLAYER_STRENGTHParameter: Layer Blending
ID_FIELDLAYER_BLENDINGMODE