The instructions on the Pushwoosh site shows crudely how to install this library for Xamarin. There are different reasons why this won’t work, but the most annoying results (on iOS 8+) in pushManager being null without any errors or issues.
One reason for that Pushwoosh_APPID is missing from the Into.plist.
The other issue, when null still appears is something I have not figured out. Simply removing all the references from the project and recompiling works sometimes; clean all doesn’t etc. It seems to be a common issue (happens often), but no-one so far produced exact steps to fix it.