To use, you must set the velocity value of the first and last notes, either manually by right clicking on the note and going to properties, or the preferred way, but simply using one of the dynamic markings such as Pianissimo, Forte, Mezzoforte, etc.
Select the notes from FIRST to LAST (an 'extended' selection, marked with a blue rectangle), then just click on the plugin and it will automatically set the velocity values. The only velocity values that are important are the first and last, the ones in between will automatically be changed regardless of current value.
It can go either way, set the first note louder, and the last note more quiet, or vice versa, and it will automatically increment the velocities up or down accordingly.