This update site contains the following CDT releases for Ganymede based Eclipse. The Ecilpse Platform Runtime Binary version 3.4 is the minimum required runtime to run CDT 5.0.x.
Enter the URL for this site in the update manager to download the CDT features
The CDT master update archive is available for each release as well. Download the zip file and enter the path to it in the Software Update as an Achived update site to install CDT. Please note that unzipping the master zip file into the eclipse directory will not install CDT properly.
If you're curious, the site.xml file is here.