This is, perhaps, a case where I should have been unambiguously clear: you generally cannot put the ampersand in as a discrete character when it is followed by a non-space character.
In other words, Z&A, Z&C, ... Z&Z, as well as Z&1, Z&2 (and a few others) cannot be used to create a header text entry that will display as Z&A, Z&C, Z&Z, Z&1, Z&2, etc. (There actually are a few combinations that can be used -- Z&B, for example -- but the vast majority cannot be used.) You could put "Z & A" (omit the quotes, note the spaces) but the question you posed asked to display "Z&A".
Since that isn't possible, I gave you the instructions on how to do it using a work-around. I did that by showing you how to store the information (using the "Author" field in File | Properties | Summary-tab) and putting the info into a field there.
Then I showed you how to display that information in the header (File | Page Setup... | Header-tab). The footer is similar (File | Page Setup... | Footer-tab).