It seems that the IB object templates in XCode 6 beta are still creating old-style objects (UIWebView for iOS and WebView for OSX). Hopefully Apple will update them for the
Not sure if this helps but I solved the problem for me by including the WebKit framework for the target. Don't embed it just link the reference. I still use the WebView object in IB and drop it on the ViewController I'm embedding it in and I've never had a problem...
I've worked on 4 WKWebView MacOS projects now and the WebView has worked properly in each project.