Rendering->Effects->Effects Tab->Add...
Add blur then scroll down and go to the Blur Parameters rollout and set blur only to work on Luminance.
Set Brightness to 100%, Blend to 60%, Min 60%, Max 100% and Feather Radius to 100%. Go from there.
Use the "Update Effect" to update the effect whenever you change a value.
yeah..
no.
i didnt do that.
what i did was..
i went to the utilities tab.. the one where you fin the polycount counter and whatnot. and then i added a new button. i added the tool Scene Effect Loader.
With this tool you can add post effects to the viewport.
load the effects which are under maps\fx\scenes on your max directory.
then just activate the effects ticking the checkbox that says effects. if you wish to edit the fx file. just double click the effect on the list. and bam!