It will work in versions before 6 but not later. Do only "does" but produces no output. In versions of Mathematica before 6, the actual picture produced by Plot was a "side-effect", but since version 6 the picture is actually output and since Do returns no output you get no picture.
There are two solutions.
1. The bad solution. Make it work just as before version 6. Print works by "side-effect" so just insert print into your code like this: