In order to contribute to the Acceleo project, first clone the git repository of the project. Once you have the source code, download a classic version of the latest release of Eclipse and install the following tools:
From the official update site of your Eclipse, http://download.eclipse.org/releases/<release-name>:
Eclipse Modeling Framework SDK
From the Eclipse marketplace:
Import in your Eclipse the projects located in the plugins repository to retrieve the main bundles of the Acceleo projects.