Does the apple watch link with the Myfitnesspal app?

Yes it does. In the my fitness pal app under settings you should see a section for devices. Find the setting and allow access to the Health app. Then go into the Health app Settings. At the bottom you will see a option for sources. Turn all categories on.

You would have to ask in the Myfitnesspal support website:

http://myfitnesspal.desk.com/customer/portal/questions/16349148-apple-health-and -apple-watch

Its their app, so they are the best to ask about its functionality with the Apple Watch and its specific Watch App.