A Milestone is an activity without duration (often used to signify the start, end, or handoff of a series of related activities). To create a milestone the first step is to click on the blue plus symbol at the top of the Gantt Chart section.
A new activity window will popup. Click on the type dropdown and select milestone. Then enter the milestone information. Name, Functional Area, and Date & Time fields are required (Note that a milestone only has one date & time selection, therefore does not have duration).