So I ran to Barry's book in Part XII - DCS Smart Device Application. Page 209 discussed "connect additional WiFi Interface units ...", but in bold type Barry notes "Do not press the WPS button!" Barry: since this is all new stuff, does MTH direction make sense?
First, never "run to my book", rather, just walk briskly.
Second, if you re-read the instructions on page 209 of The DCS Companion 3rd Edition, the bold text actually states:
except omit step #4 - do not press the WPS button on the router!
Which makes sense, since there is no router involved - the first WIU takes the place of the router and it doesn't have a WPS button.
In The DCS WiFi Companion 1st Edition the instructions on pages 44-45 have been rewritten. While they provide the same directions, they are more extensive and somewhat clearer.
The 2nd edition will discuss the DCS Explorer in detail. Unfortunately, it was only a gleam in Mike Wolf's eye at teh time the 1st edition of the book was published.