How to Navigate site. Files command to open up has been greyed-out!

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I am currently enrolled in one of O'Reilly School of Technology classes. I am currently having difficulty in navigating the server/lesson. When I hit the file button to open up lesson two, php files, the open file button was greyed-out! Is there anyone there that could walk me through, answer questions on HOW to open each lesson? The help videos were NOT helpful. Greyed -out commands mean either I am doing something wrong (wrong sequence of commands) OR my computer has been hacked into (I have a Mac computer) Is there someone I could talk to over the telephone that could help me navigate, understand how to proceed and open each lesson plan?