CreateUser namespace problem

I have the following coed in my project,
 
MembershipCreateStatus createStatus;
MembershipUser newUser = Membership.CreateUser(Username.Text, Password.Text, Email.Text, passwordQuestion, SecurityAnswer.Text, true, out createStatus);
switch (createStatus)
{
case MembershipCreateStatus.Success:
CreateAccountResults.Text = "The user account was successfully created!";
break;
case MembershipCreateStatus.DuplicateUserName:
CreateAccountResults.Text = "There already exists a user with this username.";
break;
case MembershipCreateStatus.DuplicateEmail:
CreateAccountResults.Text = "There already exists a user with this email address.";
break;
case MembershipCreateStatus.InvalidEmail:
CreateAccountResults.Text = "There email address you provided in invalid.";
break;
case MembershipCreateStatus.InvalidAnswe ...