Comment 1 for bug 685845

Hello Vittorio,
same here (with original name "Mixxx.app").

Create a blank file "qt.conf" in your "Mixxx.app/Contents/Resources/" directory.
Insert the following 2 lines into "qt.conf" and save.

[Paths]
Plugins = plugins

More:
https://answers.launchpad.net/mixxx/+question/92851
http://mixxx.org/forums/viewtopic.php?f=3&t=1790&p=6632