To call this method, the “Membership.Provider” property must be an instance of “ExtendedMembershipProvider”

Try setting the following up in your web.config within the <system.web> node:

    <roleManager enabled="true" defaultProvider="SimpleRoleProvider">
        <providers>
            <clear />
            <add name="SimpleRoleProvider" type="WebMatrix.WebData.SimpleRoleProvider, WebMatrix.WebData" />
        </providers>
    </roleManager>
    <membership defaultProvider="SimpleMembershipProvider">
        <providers>
            <clear />
            <add name="SimpleMembershipProvider" type="WebMatrix.WebData.SimpleMembershipProvider, WebMatrix.WebData" />
        </providers>
    </membership>
    <sessionState mode="InProc" customProvider="DefaultSessionProvider">
        <providers>
            <add name="DefaultSessionProvider" type="System.Web.Providers.DefaultSessionStateProvider, System.Web.Providers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" connectionStringName="DefaultConnection" />
        </providers>
    </sessionState>

Sounds like SimpleMembership isn’t recognized when you go to use it.

Also, it may be worth including, the WebMatrix.webData library can be installed via NuGet:

PM> Install-Package Microsoft.AspNet.WebPages.WebData