can you repeat your question and restructure it differently? what is "mixxx"? is it a GUI application or a terminal one?. Why do you want a login manager to start it up?
Different desktop environments use different ways to start programs at log in. If you want this application to start up after you log in, mention the desktop environment you use and somebody who know how to do it will help you out.
If it is a terminal based program and you want to start it up as some sort of a service running as root then you can start it up by adding appropriate line in "/etc/rc.local" text file.
Basically, you can start this program before a desktop manager starts or after it has already started and the desktop environment is somewhere close to finish logging in a user.