Hi Aldo,
There are a few unknowns here. I don't know how Max handles the key frames when motion blur is turned off. You should also be able to choose how the shutter is offset in Max. Sometimes you just have three choices: centred, start, end.
If Terragen matches when you set the shutter offset to -0.25 and the blur length to 0.25, that means that the Max render is rendering as if the key frame is at the end of the exposure (I think), which seems unusual. Will you be rendering with VRay? I'd recommend setting the camera to "centred" in Max/Vray, then in Terragen set the shutter offset to minus the motion blur length divided by 2. That should match. In any case, there should be a way to get them to match almost 100% whatever settings you have in Max, if I know the setting you use. The only big problems occur if you have multi-segment motion blur or non-linear blur which Terragen doesn't support and/or if you have very high curvature between key frames but I don't think that's what's happening here.
Matt