Make sure that this file exists: ./Class/galaxy/Univers.class (starting from your basedir where build.xml is located)
I don't use a Mac, so I have no idea what the Finder screenshot is telling me (I'm not even sure that is from a Mac, I'm only guessing) - exactly what directory are the displayed Univers.class and Univers.java files located? I would have expected them to be located at
By the way, what is the package declaration within Univers.java? I am assuming it is "package galaxy;" based on the definition of MainClass in the build.xml.