Upload the upload directory (the directory named dark_castle). Once that is in your root, go to your admin panel>styles and templates>download/upload styles>Import Style XML File. Browse until you find the Dark_Castle.xml file and select it. Then click on import. It is uploaded.
You can go into your vBulletin Options and make it your default or let the users choose if they want it or not. Mine is set as default...
You should be done...
mine -
http://softhacks.info