I passed by http://www.theserverside.com and saw the release of Middlegen 2.0 VO!

Can someone please tell me the diference between this version and the R3 version (chich is the one I'm using)?

Check the notes that are supplied with the R4 release. I will continue to release Hibernate builds.

I've downloaded R4 but can't find the changelog.
Where can I get the changelog for R4?

The changes are documented as a link on the source forge download page, eg, the title of the download. To make it easier here is what it said:

This is the fourth release of the Middlegen with hibernate plugin. It includes enhancements and fixes.


Changes:
o Middlegen 2.0 vo (final) - Middlegen 2 is no longer beta software.
o The Code has been moved to CVS Head so updates should be smoother now.
o A new full release has been added to the Middlegen sourceforge download.
o XDoclet 1.2 final jars (a subset) included.
o Fixed Minor typo for XDoclet meta Tags for the class defintion generation.
o Added XDoclet meta tags for elements of the compound keys.
o (Re)Introduced the Unique constraints detection in Middlegen core.
o Mappings will be correctly generated with the unique attribute.
o Added inherit="false" to a number of the meta tags for some standard meta attributes and XDoclet meta attributes.
o Fixed issue where sometimes the generator assigned was being reset in the GUI.
o Changed one-to-one mappings to not use contrained="true" for nullable foreign keys.
o Fixed one-to-one constrained="true" moved it to the otherside of relationship.
o Added the ability to provide custom defaults for Type selection.
o This option allow the developer to provide a JavaTypeMapper plugin for the plugin.
o The custom mapper is introduced through the Ant script - see docs.
o Provided a much improved type selection module (HibernateJavaTypeMapper) using the new JavaTypeMapper plugin mechanism.
o The class provides an example on how you can build your own custom JavaTypeMapper.
o It provides better selection of selection of default types and considers if the SQL type is a nullable field.
o Preferences file is now located in Src rather than Build (as it was in some previous releases).
o Other minor changes