Unfortunately variables aren't global.  They're scoped to the page you're working on.

For now you have to build your own class with a method that sets up your class and call that from every page.

Drop a file called DALHelpers.cs into the App_Code in the root of your app.  Put this in the file...

public static class DALHelpers {
    public static DAL CreateDAL() {
        DAL mydll = new DAL();
        mydll.ConnectionString = (string)ConfigurationSettings.AppSettings["ConnectionString"];


        return mydll;
    }
}

From any page now you can do this...

@{
    var mydll = DALHelpers.CreateDAL();
    var data= mydll.getfunction(year, username, out ex);
    Response.Write(data);
}

Not ideal, but should work for you.  In the next release we'll allow you to put an @functions {} block into a file in App_Code and the methods will be globally accessible to any page.  That should simplify it a bit.