Well,
 Technically Brix and Plato are two ways of measuring the same thing.  Both measure the percent of sugar by weight in a solution.
 The confusion comes in when you use a brix refractometer because a refractometer is calibrated to measure sugar (sucrose) in water, not complex sugars in wort or sugars combined with alcohol.  That's why you need to use the refractometer tool in BeerSmith to convert your brix measurements to OG/FG and so on.  The differences may be small when measuring OG, but once fermentation starts you have alcohol in the solution which is much less dense than water - this basically throws the whole thing off unless you adjust using the refractometer tool.
 I hope this helps a bit...
Cheers!
Brad