If you have InfraWorks 360 installed, start the application and then sign in using your Autodesk 360 account credentials.
InfraWorks 360 Home will display after you sign in.
To sign out, toggle the menu underneath your profile picture and name in the Utility Bar, and select Sign Out. 
You must be signed in to InfraWorks 360 to view, filter, and download cloud models, comment in the design feed or collaborate in design groups.