From the File menu choose Open Project or from the Project Manager click on the Open tool button and choose the select selected file to open or from the Project Manager Tab click Open Project.  This will open the Project tree in the right hand panel and open the main project source in the editor.  

Working with a Project

Other files in the project will be listed in the Project tree and can be opened by double clicking on the relevant node.  Files which can be opened in the IDE will be opened in the relevant editor/viewer.

Files can be added to and removed from the project by right clicking on the relevant file or by using the appropriate button on the Tool Bar.

You can only change the Processor or Xtal value of an existing project by changing the source. The new processor and Xtal will be picked up next time you open the project and you will be given the choice to accept the changes or retain the old values in the Project File.