If you have an existing Eclipse installation and simply wish to install Acceleo in it, you can install it from the Update Site, available here https://download.eclipse.org/acceleo/updates/nightly/.
When installing the update site you must ensure that "Contact all update sites during install to find required software" is checked and that in the "Manage…" dialog, the "Latest Eclipse Release" update site is checked.
For those of you who need to retrieve the source code of Acceleo, it is available on Git: https://git.eclipse.org/c/acceleo/org.eclipse.acceleo.git/.