Projects created on one platform cannot be moved automatically to another due to where they are stored.
Projects created via the MILK Books website are stored on our server which means that you can access them from any device so long as you have signed into your account. Projects created via the MILK Books Photo App extension for Mac are stored locally on your device so are not uploaded to MILK until you place your order. The benefit of this is that you can create while offline, however, it also means that MILK does not have access to your files so we are unable to recreate your project online.
Since the projects are stored in different places according to which platform you have used, projects cannot be moved between the website and Apple extension, and an account on the Photos App Extension cannot be linked to an account on the website. To move your project from one to the other, you will have to manually recreate your project.