The preferred way to download the plug-in is via the Eclipse update manager. Under Help > Software Updates > Find and Install... chosse Search for new features to install and click next. Then choose New Remote Site... and enter as URL
http://c4jplugin.sourceforge.net/update
Click Finish and go through the dialog which follows.
You don't have to install the runtime plug-in separately. It is available from the same update site and will get installed automatically.