I just found an answer to this in a google forum, thought I'd post it here because this post comes up at the top of search results.
Quoted from the other forum: "Struggled with this as well. There is a dimension style palate found under the window menu (mac anyway) that controls architectural feet and inches when you pull a dimension. Non intuitive as the slashes and line weights are controlled under yet another shape style palate. The document setup directions parroted hear [sic] are a bit of a snipe hunt given your question."
So I found that palette and it, too, had inches grayed out. But when I selected a dimension line I'd already placed in my layout sheet, and then changed the drop down menu from 'fractional' to 'architectural', it switched from 144" to 12' 0". Yippee.