1. Download Eclipse

2. Add MDT project
Help > Software Updates... > Available Software > Add Site...

http://download.eclipse.org/modeling/mdt/updates/releases/

see http://www.eclipse.org/modeling/mdt/updates/

choose "MDT UML2 SDK 2.1.1"

or alternatively

eUML2 at http://www.soyatec.fr/euml2/installation/manager.ganymede.php

http://www.soyatec.com/update/ganymede/