You have to make sure that the circle is at all times covering the whole width of the letters` stroke. Move the timeslider back and forth and view at 200 or 400% to check. Adjust the path if necessary.
Once you got the perfect path (which really is the only difficult part of this whole tutorial), duplicate the solid layer and scale the circle up a bit to fit the outline strokes. Select the n-fill-in layer and the n_fill-in_mask-layer. Press Control-Shift-C to turn them into a pre-compositon, check "move all attributes into the new composition". Do the same with the outline and outline-mask layer. In the project window give these compositions names like n_fillin_animatedmask and n_outline_animatedmask. Open the compositions. Make sure that in both comps the solid layer is at the top, then use that layer as the bottom layers track matte (pic 12). When you move the timeslider you should see something like in pic 11.