Using the Microsoft Project Integration
Select the project you would like to view.  All you need to do once you have logged in to Microsoft Project Online is to click the title of the project.  You will need to put the project into edit mode.  It will open in read-only by default.  To do that, click the "Project" tab.

Using the Microsoft Project Integration

Microsoft Project Online - Accessing and Updating Existing Projects

Using the Microsoft Project Integration