原文地址：http://stackoverflow.com/questions/35802657/unity-5-3-3-ui-does-not-exist-in-namespace
问题：Thetype or namespace name'UI'does not existinthe namespace'UnityEngine'(are you missing an assembly reference?)
解决方案：这个问题，是由于你的上次发布版本的时候用的webPlayer方式发布的原因导致的。
This happens when your last build target was "Web Player".
1 Open: File->Build Settings...
2 Choose PC, Mac & Linux Standalone
3 Build your game - during this step the VS solution settings will be fixed
