Sort files according to kind in Office for Mac save dialog
The answer is covered in this microsoft forum discussion, but in essence what you need to do is 


switch to list view in the file dialog
be sure "Kind" is one of the columns shown

if it isn't, hold "control" and click on the bar with column names
select "Kind" to show that row

then you can click on the "Kind" header to sort, and all folders will be together.


This doesn't put folders at the top though. If you really want to do that, you can read the solution someone devised there. The labelled every folder, then sort in list view not by kind, but by label.