When I went to my layers panel the Master Page was already unlocked, but I could not edit the Master elements on an individual page. So I created a second Mater (Master B) and added the element I wanted to appear. I then went back to the Layers panel and "right-clicked" on Master Page B. From there, I selected "Edit Detached," made my changes and clicked the red "Finish" button that appears at the top of the window and saved my work. It worked perfectly! Now, Master A applies to all pages, Master B applies to a few of the pages, and using "Edit Detached" (from the Layers Panel), I was able to make modifications that Master B added, to individual pages.