Hi,
2d toolkit isn't compatible with the new UI system out of the box. I am happy to add support for tk2d sprites in the new unity ui, but there really hasn't been too much interest in this. If there is enough interest I will add it and officially support, but to do it "properly" and maintain it is a significant amount of work.
I do remember someone posting a solution on github that worked in a lot of cases somewhere, but I don't seem to be able to find it... If you can't find it and really need this, let me know - I can make a simple adapter that works in lot of common cases (not all) really easily.