Creating a diagram from source code
If you want to create a diagram from a package which already exists in your Java code, you need to select the package, click the right-hand button of your mouse and choose the option "Add Package to diagram".
Then all the objects the package contains are added to the diagram you're working with at the moment. You can arrange those objects in the way that suits you by clicking on them and moving them around in the diagram via drag and drop. An automatic layout is not provided at the moment.
In the same manner you can add a project to the diagram by clicking on the option "Add project to diagram" or add a class to the diagram with the option "Add to diagram".
Now if you continue programming your Java code and you add, for example, new attributes or methods to your classes, these new elements will automatically be added to your diagram. Thus consistency between the code and the diagram is guaranteed.