Click the gear-shaped icon in the top-right corner of the page to do so. A drop-down menu will appear. It's a link in the drop-down menu. Doing so opens a pop-up window. You may not need to scroll down to see this link. It's in the upper-left side of the window. You'll find this in the middle column of options. Click the "Add a sender here" text box at the top of the "Blocked senders" section, then type in the person's email address. This is to the right of the text box.

Summary:
Open the "Settings" menu . Scroll down and click View all Outlook settings. Click the Mail tab. Click Junk email. Enter a contact's email address. Click Add.