If you have installed iHub, make sure you are using the commercial version of the designer, Analytics Designer, to modify the report designs. Otherwise, you may have incompatibilities, especially with HTML5 charts.
From Analytics Designer, the easiest way to import a design is to create a server profile that will connect the Designer with iHub. You can do this by going to the Server Explorer window in the Designer, right-click on Servers, choose New Server Profile, and provide the connection information to iHub. Once done, you will be able to see the content of iHub in the Server Explorer and simply drag-and-drop report designs from iHub into your BIRT project.