You must have downloaded BootStrap from twitter in you MVC project. Also you need to insert the common libraries of bootstrap in your main page (normally is your /Shared/_Layout.cshtml)
<link href="/Content/bootstrap.min.css" rel="stylesheet" type="text/css" /> <script src="/Scripts/bootstrap.js" type="text/javascript"></script>
Before start to develop, check in your project references if SushiMVC is correctly installed the htmlHelpers.dll reference.