If this still fails, make sure you have no update site "registered" with this URL :
- Window > Preferences
- Go to "Install / Available Software sites"
- Copy paste the URL above in the filter list
If anything shows up, select it and hit "remove". Then, retry the installation procedure. If it still does not work, You'll probably have to download a new Eclipse, as this update site is functionnal (I just installed EMF Compare 2.1M5 from it in one of my Eclipses).