This has been discussed before!!!! DO SOME SEARCH BEFORE POSTING!!!!
To enable the CustomView in the designer you MUST have a CustomView lib or class in your project!
Personally I'm fine with such questions. The OP has started a new thread with a proper title and a short question.
As this is a new feature I'm not sure that it is very simple to find the answer.
Later when someone else searches for customview grey then this post will be the first search result and it will be helpful.