Mixcraft project files (mx4, mx5, mx6) do not contain any audio (although they do store MIDI data). Your recordings and audio files are stored separately. This means that when you transfer your Mixcraft projects from one computer to another, you can't just copy the .mx6 files, you have to also copy all the associated .wav and any other audio files. Mixcraft 6 has a feature that makes this task much easier.
Create an empty folder for each project in Windows. In Mixcraft, load up a project, then click on the File menu and select "Copy Project Files to..." Choose a folder or zip file and click OK to transfer the sounds. Once you have your projects organized in this way, it's easy to zip them up and transfer them to the other computer.