Creating milestones
There are several ways to make a new milestone in task view:
Create a new task, then right-crick on the task to use the context menu and change its Type to Milestone.
Click the Add Milestone button 
in the toolbar. (Right-click on the toolbar then choose Customize Toolbar to add the button, if it's not already showing.
in the toolbar. (Right-click on the toolbar then choose Customize Toolbar to add the button, if it's not already showing. Choose Add ▸ Milestone from the action menu below the task outline.
Use the Project Milestone Inspector to manage your milestones and customize them.