In many cases, importing the .xml file does not mean that your site will look like the demo out of the box. The .xml file will import dummy content, as images, posts, and pages, but in many cases (for example when the theme uses a widgetized homepage) you will need to manually configure the widgets and the menus because this information cannot be “transferred” via the .xml file.
Please read the related documentation for your theme, as it gives you detailed instructions on how the widgets, menus, settings and the theme itself works. Once you get the logic behind it, you will be able to set up everything in no time.