This didn't work for me (Word 2010, Win 7, 64 bit) but this does;
Enter Word art text as you wish, then
- select the text you wish to rotate
- the Drawing Tools, Format ribbon will show;
on this ribbon go to Wordart styles and left-click on "text effects"
- this brings up a sub-menu, hover over "3d rotation" and left click on "3D rotation options"
- select "X" and set it to "180"
-left-click "close" to action this on the selected text.
It's just a shame the "flip horizontal' command doesn't work on the text, just the bounding box...