You need to understand, that OSM.org uses a pure rendering engine called Mapnik. Tilemill is a wrapper around Mapnik by simplifying stylesheets.
As @scai say, you need to create a stylesheet that renders a label for housenumbers. I'm not familar with tilemill to tell you what need to be done, but usually this consist of
- Adding a label symbolizer that looksup the addr:housenumber=* tag and prints the value in a appropriate font and size
- (creating fake nodes that deal with the placement of that labels within building outlines etc.)