Connection string for database connection

Actually its my classroom application but any i want to be good in all.
my requirement is i need to create 3 layers for simple application..
everything is fine but some new problem araise
now i got the following error
Error Summary
HTTP Error 403.14 - Forbidden
The Web server is configured to not list the contents of this directory.
Im copying all my application code here.
DataHelper.cs
<pre lang="c#">public class DataHelper
{
public DataHelper()
{
//
// TODO: Add constructor logic here
//
}
public static DataSet GetData(String sqlstat)
{
String str = @"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\inetpub\wwwroot\WebSite\Dbweb\App_Data\pubs.mdf;Integrated Security=True;User Instance=True";
SqlConnection cn = new SqlConnection(str);
SqlDataAdapter da = new SqlDataAdapter(sqlstat, cn);
DataSet ds = new DataSet();
da.Fill(ds);
return ds;
}
public static void Execute(String sqlstat)
{
String str = @"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\inetpub\wwwroot\WebSite\Dbweb\App_Data\pubs.mdf;Integrated Security=True;User Instance=True";
SqlConnection cn = new SqlConnection(str);
SqlCommand cmd = new SqlCommand(sqlstat, cn);
cn.Open();
cmd.ExecuteNonQuery();
cn.Close();

}

}</pre>
BLJobs
<pre lang="c#">public class BLJobs
{
public BLJobs()
{
//
// TODO: Add constructor logic here
//
}
public DataSet GetJob()
{
return DataHelper.GetData("select * from Jobs");

}
public void AddJob(String desc, int minlvl, int maxlvl)
{
DataHelper.Execute("insert into Job values('" + desc + "'," + minlvl + "," + maxlvl + ")");
}
}
</pre>
and this is my webform

<pre lang="c#">public partial class Dbweb : System.Web.UI.Page
{

BLJobs obj = new BLJobs();
protected void Page_Load(object sender, EventArgs e)
{

}
protected void LinkButton1_Click(object sender, EventArgs e)
{
GrdView.DataSource = obj.GetJob();
GrdView.DataBind();
}
protected void LinkButton2_Click(object sender, EventArgs e)
{
obj.AddJob(TxtDesc.Text, int.Parse(TxtMinlvl.Text), int.Parse(TxtMaxlvl.Text));
Lbl.Text = "Record has been Added";
}
}</pre>