Two things occurred to me after sending my reply;
1- The commands only work for *Linked* text boxes, not for general
navigation, and
2- The button commads are worded differently: Next Text Box & Previous Text
Box respectively.
One option if you want to navigate from one unlinked box to another you can
select the box, Bookmark it then use the Go To feature to move from one
Bookmark to another. This has to be done on a per document basis, however.