Creating milestones
There are several ways to make a new milestone in task view:
Create a new task, then right-click on the task and use the contextual menu to 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 there.
in the toolbar. (Right-click on the toolbar then choose Customize Toolbar to add the button, if it's not already there. Choose Add ▸ Milestone from the action menu below the task outline.
Use the Project Milestone Inspector to manage your milestones and see the critical path leading to each one.