Blender includes a large list of import-export options. By default a lot of these aren't enabled, import-c3d being one of them. I count 33 import-export addons in the current version with another 17 available that aren't normally included with an official release. Only 9 of these are enabled by default.
To enable an import addon you need to turn it on in the preferences - File->User Preferences under addons.
Click the Import-Export button (1) to list all import addons or enter a search term (2) to find by name. Then enable the addon by clicking (3) this will make the option available under File->Import.
As for the download link - projects.blender.org has just recently been replaced with develop.blender.org from the move from svn to git. Links in the wiki still need to be updated.