Erratum—
Some of the purl dots are missing from Chart E.

On rows 2, 6, 10, and 14 add knit stitches (these are WS rows, so the stitches will appear purled on the RS) within the center cables, following the pattern established on rows 1, 4-5, 8-9, and 12-14.

This sounds a bit more complicated than it is. Just look at Chart E and compare it with Chart A—you’ll see what needs to be done.