Mendix uses the Java DecimalFormat class in the background: http://docs.oracle.com/javase/7/docs/api/java/text/DecimalFormat.html
You can use all paterns described on there.
This page describes for a #: zero shows as absent. That means if you want to show a zero value as 0.00 you would have to use a pattern such as this: "##,###,##0.00"