A family of Microsoft word processing software products for creating web, email, and print documents.
Charles is correct that if the instructions you were attempting to follow refer to a "Design Mode" they are for the PC version of Word... Word for Mac has no Design Mode, but I believe the real issue may be with your expectation :-)
Help Text does not display in the Text Box Form Field to which it pertains, nor does any sort of tip box appear "on rollover". Whatever is specified as Help Text appears at the left end of the Status Bar when a user tabs into the field of a Protected Form.
If you want the Text Box, itself, to display some sort of content type it into the Default Text field in the main Field Options dialog window.