cannot enter users management in ASP.NET Configuration (aspnetdb database)

Hello,
Please help in the following problem. (I translate texts and messages from
Polish to English, so there may be some differences).
I have created ASP.NET application with aspnetdb database and I have created
one user: demo. The problem is that when I enter ASP.NET Configuration (from
Visual Studio Express Edition) and I choose 'Security' tab, then 'Manage
users' I receive an .aspx page with error:

Generating user instances in SQL Server is disabled. Use sp_configure 'user
instances enabled' to generate user instances. in
System.Web.Administration.WebAdminPage.CallWebAdminHelperMet hod(Boolean
isMembership, String methodName, Object[] parameters, Type[] paramTypes) in
ASP.security_users_manageusers_aspx.Page_Load() in
System.Web.Util.CalliHelper.ArglessFunctionCaller(IntPtr fp, Object o) in
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Obje ct sender,
EventArgs e) in System.Web.UI.Control.OnLoad(EventArgs e) w
System.Web.UI.Control.LoadRecursive() in
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint,
Boolean includeStagesAfterAsyncPoint)

Executing
sp_configure 'user instances enabled'
in SQL Server 2005 Express Edition (using aspnetdb database) doesn't help -
there's still the same error.
So far I had no problems with my aspnetdb database.
Do you know what is going on? Why the error occurs? How to solve the problem
to enter users management?
Please help.
Thanks in advance.
/RAM/