How can I use it?
[>] motion blur?
Thank you!
you must animate your scene in some way for it to work. Put your objects on a path, move the camera, something… hope this helps. Follow the documentation to animation if you don’t know how to do that.
You might want to do post pro motion blur. It’s not great quality but it’s faster than Blender. If you have an animation with 25fps and you turn on motion blur, Blender will render 7 or 8 extra frames per frame to do the blurring. You might be able to do post-pro blur in GIMP but I reckon you probably need a compositor.
The [Bf] slider right below the [MBLUR] button under the Render Buttons sets how many frames to blur ahead of the current frame being rendered. This is called the Motion Blur Factor and the default is half a frame. This means, if it were a real camera, that the lens would be open for half a frame.
The numbered buttons Below OSA not only effect how many antialiasing samples to take, but also the number of motion blur samples to take. So if you select 5, it will render 5 “sub-frames” and then composite them together to create a blur effect. The bigger your blur factor, the more samples you will want to take.
I’ve also found that when you use motion blur, you sometimes do not need to have OSA turned on, which can save rendertime. And please, unless you’re going for a drunken look, don’t put the blur factor over 0.5 
Just for precision sake, I’m pretty sure it’s the number of frames after the current frame. Meaning that on Frame 1 with a bf value of 0.5, you’d render from Frame 1.0 to Frame 1.5.
Could be wrong, but I just verified to be sure and it very well seem to work this way.
Martin
If you go ahead on a timeline, which way do you go? Sorry if it was confusing.
Well well, since we are both right…
Martin