To turn ON MB in Kray simple Turn ON in Camera tab Motion Effect/Motion Blur.
It works only with Full Screen AA in Kray (more passes - better quality)
There are not MB special buffers in Kray - it would be nice use it in post process.
In K3 will be special MB done by Kray (I think by vector motion so it will be less dependent from AA level). Now MB is just emulated from LW.
However I'm getting blur on objects that are moving close to the camera.
Maybe you have turned ON DoF?