I think I see what's happening. Make sure LocalDataCredentials is instantiated with the correct values before assigning to Credentials property of the PinAuthorizer. Something like this:
Credentials = new LocalDataCredentials
ConsumerKey = "consumerKey",
ConsumerSecret = "consumerSecret"
I still have a sequencing bug where the credentials don't get passed to dependent objects if you change them after the Authorizer is constructed. Here's my blog post on one way to do this in Windows 8:
BTW, my latest bit of code includes a WinRtAuthorizer. I haven't documented or blogged about it yet, but it's very nice.