To insert a simple border graphic around the whole layout (page), use the Insert > Graphics and Text option, selecting Rectangle.
Once you've drawn it, you can change the properties to make it hollow (No Fill) and the outline to suit your border requirements.
Once you've drawn your rectangle, I'd suggest making sure it's at the bottom of the Drawing Order list in the Table of Contents (under all your other elements and map frames) so you're not constantly selecting it when clicking on the layout.
Lindsay Raabe
GIS Officer
Forest Products Commission WA