To work with Eclipse 4 application models, it is necessary to install the E4 tools to allow the application XML to be edited through an editor. These are not shipped with all the Eclipse packages, and must be installed separately.
Add the following update site by going to the Preferences menu (from the Eclipse menu on macOS or the Window menu on other operating systems), followed by navigating to Install/Update | Available Software Sites, then clicking on Add..., and entering http://download.eclipse.org/e4/updates/0.17.
Note
Some of the E4 tools are available in the Eclipse Mars update site, but not all of them. There may be updates available or additional releases of the E4 tools site; check out http://download.eclipse.org/e4/updates/ and look at the directory contents to find a later release for later versions of Eclipse.
Click on OK on the Add Site dialog and OK on the Preferences page to add it. Once added, navigate to the Help | Install New Software...