You need to make sure service account 'IIS APPPOOL\ASP.NET v4.0' the app is running under has an access to SQL server/database. You may either add 'IIS APPPOOL\ASP.NET v4.0' user to the database, or run the app under another user (e.g. your windows account). Another approach is to create new account within SQL server an specify it in web.config for CMSConnectionString along with SQL server authentication mode. I'd also like to admit this is not Kentico related issue, this is something you have to do for any ASP.NET application.