It looks like you are running in a MVC 1.0 context, since the class that seems to be missing was added in Mvc 2. Could you look if you have the System.Web.Mvc.dll file in the bin folder of your site. If this is the case, what version is it? Another thing you could check is that you have a binding redirect in web.config for the Mvc dll to version 2. Something like:
<assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="188.8.131.52-184.108.40.206" newVersion="220.127.116.11" />
Seems there was a problem with the Visual Studio installation.
I tried another virtual machine, and it worked fine.
I'm trying to install the brand new CMS 6 R2 that was released yesterday - and as the lazy guy I am, I'm trying the Express install option.
I got an error saying something about missing something and MVC, so I installed MVC first, and then tried again.
Well... Same problem:
Error - Method not found: 'System.Web.Mvc.ModelValidatorProviderCollection System.Web.Mvc.ModelValidatorProviders.get_Providers()'.