I guess the theme is not open source but unless it has any different installation process, once you put the directory that contains the qa-theme.php file inside the qa-theme directory it should already be installed. In other words, the path for the qa-theme.php file should be something like qa-theme/FlatBox/qa-theme.php.
Once you have put it in there Q2A will detect it but you still have to select it. So navigate to admin/general and select the FlatBox theme from both comboboxes: Site theme and Theme for mobiles.
