I have downloaded the code files for "ASP .NET Website Programming, Problem-Design-Solution, VB.NET edition", and I'm trying to iron out the initial errors. I currently have an error message which is not mentioned in the readme file.

The message is reproduced below, can anybody explain to me what the error is about and how to solve the problem please?
Any help would be much appreciated.
Thanks in advance.

An unexpected error has occurred on this page.The system administrators have been notified.

The error occurred in:
http://localhost/thephileVB/Default.aspx

Error Message:
Object reference not set to an instance of an object.

Error Stack:
System.NullReferenceException: Object reference not set to an instance of an object.
   at Wrox.WebModules.AdsManager.Data.Advertisement.Retr ieveAd(Int32 advertisementId) in C:\wrox\ThePhileVB\WebModules\AdsManager\AdsManage r.Data\Advertisement.vb:line 39
   at Wrox.WebModules.AdsManager.Business.Advertisement. .ctor(Int32 advertId) in C:\wrox\ThePhileVB\WebModules\AdsManager\AdsManage r.Business\Advertisement.vb:line 47
   at Wrox.ThePhile.Web.Controls.User.SiteFooter.Rotator _AdCreated(Object sender, AdCreatedEventArgs e) in C:\wrox\ThePhileVB\Controls\User\SiteFooter.ascx.v b:line 67
   at System.Web.UI.WebControls.AdRotator.OnAdCreated(Ad CreatedEventArgs e)
   at System.Web.UI.WebControls.AdRotator.OnPreRender(Ev entArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain()

Don't worry!
I had missed out the changes to one of the config files.
I think that problem is sorted now