Hi , i am trying to install DNN 7.0 community version in my virtual machin Windows server 2008, i followed the installation video exactly, until i get to the step where i add the application pool user 

iss appPool/dnndev.me 

which in my case is not available, so i added the login user name for this server, also  modified the security on the folder level for all Authenticated user, also i added it as an owner to the DB

now when i run the site it will load with an error Database Connection Error

i dont know how to solve it, any suggestion

thank you for the reply, i am using SQL server managment studio 2012 with localDB lightweight version of SQL server express DB engin
so which connection string do i have to change?

<!-- Connection String for SQL Server 2008/2012 Express -->
    <add name="SiteSqlServer" connectionString="Data Source=.\SQLExpress;Integrated Security=True;User Instance=True;AttachDBFilename=|DataDirectory|Database.mdf;" providerName="System.Data.SqlClient"/>
    <!-- Connection String for SQL Server 2008/2012
    <add name="SiteSqlServer" connectionString="Server=(local);Database=DotNetNuke;uid=;pwd=;" providerName="System.Data.SqlClient" /> 
    -->

Thank you for the reply, and sorry for the delay, cuz i frustrated from this problem, so i left it with out solving it, now i am back to try again, 

so i removed this account and now i am using NETWORK SERVICE account for the site folder, Application pool, and for Database, 

now the page is loaded partially and database part show error "Database Connection Error" 

please help