I'm sorry, but Mixcraft is not one of our officially supported hosts. It might very well work any way, it's just that we haven't tested it ourselves and cannot guarantee that it will work or help you get it up and running.
See complete list of supported hosts at http://www.xlnaudio.com/en/products-add ... quirements
It sounds to me like Mixcraft is not looking in the right folder for VST plugins. You probably need to add this folder to the VST scan folders in Mixcraft: