As Bartman suggests, the problem is normally not with MC, but with the content (file, tv ....) you are trying to play, or with your hardware setup. So long as you have the monitor running at a 16:10 widescreen resolution and you try to play a movie/TV with a correct aspect-ratio tag in the file/broadcast (and MC is set to use source aspect ratio under display settings) it should appear correctly.
** The only way I know to access the aspect ratio setting is either via Theater View, or by right clicking on a playing video image ** (I cant see it under options...)
If you are trying to play TV, does your card driver have a setting for the output aspect-ratio? This may be confusing MC if it is set incorrectly. Check your default aspect ratio settings in MC. I generally find "Use Source Aspect..." is correct.
(I know this isnt your issue, but may be useful info)
For general movie file viewing:
Be sure that the movie you are playing has the correct aspect-ratio identifier (a few dont, and this can cause MC to disply it incorrectly, but is easily overridden in the display settings).
Also be sure that the movie is 16:9 and not 2.85:1 (or any of the super-widescreen ratios), this will still cause you to get large black bars top and bottom, although this is correct behaviour.
Basically if the movie doesnt look stretched in either direction, its displaying as it should.