To work with Eclipse 4 modeled applications, it is necessary to install the E4 tools to allow the application XMI to be edited through an editor. These are not shipped with the standard Eclipse distribution package, and must be installed separately.
Add the following update site by going to Window | Preferences | Install/Update | Available Software Sites, then click on Add... and put in:
Juno (4.2): http://download.eclipse.org/e4/updates/0.13
Kepler (4.3): http://download.eclipse.org/e4/updates/0.14
Click on OK in the Add Site dialog and on OK in the Preferences page to add it. Once added, go to Help | Install New Software and then select the newly added site in the list. Select the the following features:
CSS file editor
CSS spy for Eclipse 4
Eclipse 4 core tools