I had the same problem as described here http://forums.xamarin.com/discussion/31355/methods-missing-in-nsurlconnectiondelegate, I was in the middle of migrating my app hypstr (www.hypstrapp.com) to the Unified API.
The problem occurs with the NSUrlRequest, which requires an instance of INSUrlConnectionDelegate, where prior to the Unified API I was using NSUrlConnectionDelegate.
Udating my library to Unified API a couple of methods I was using before are missing. Namely ReceivedData(NSUrlConnection, NSData) and FinishedLoading(NSUrlConnection).
The solution is here https://github.com/Cheesebaron/Cheesebaron.MvxPlugins/commit/8e57a7cb53f8ec575ad9c48e478e7ab44bcb93a0#diff-d0b47785af23a5a11abceed2acd46b00R131, you should now inherite form NSUrlConnectionDataDelegate instead of NSUrlConnectionDelegate
And that's all :)