Ok, this response is somewhat late, but I may have found an answer to what seems to be the issue. I am using MS Office 2016 for mac whilst my colleagues are using MS Office 2007. Office 2016, even in compatibility mode, does not allow you to edit text boxes that have been generated with a previous version. In order to overcome this, simply select the text box by clicking on the paragraph mark behind the anchor (you should have enabled "Show all nonprinting characters") and on the "insert" tab click "draw new text box". A popup appears stating that the text box was generated in an old version and should be converted. It will show a button to convert it to the 2016 version. You will then be able to edit text.