To contribute to the ESDL language you have to install Eclipse and the Eclipse Modelling Tools which contain the Eclipse Modelling Framework.
These can be downloaded from the eclipse.org website. Easiest is downloading the modelling package directly with a new Eclipse installation, but alternatively you could also install these modelling tools as plugins in your existing Eclipse installation. Since EMF is still under active development, and ESDL is using new features from these latest releases, it is required to use the latest version of Eclipse.
Currently Photon is the latest stable release of Eclipse. You can download the Eclipse installer from the general download page and select the Eclipse Modelling Tools from the list of options after running the executable.