Good question. It is
not
the "Structural Grid" add-on, but the "Grid Tool” add-on available from the Goodies download webpage (you actually don't want the two add-ons loaded at the same time). There are instructions for it's use on the webpage as well -- but the tool can be a bit fussy (as can be witnessed in other topics on this forum).
MacBook Pro Apple M2 Max, 96 GB of RAM
AC27 US (5003) on Mac OS Ventura 13.6.2
Started on AC4.0 in 91/92/93; full-time user since AC8.1 in 2004