If you installed Eclipse from the Ubuntu repositories, your software sites list might be emty. In that case, Eclipse won’t update itself and you won’t be able to install new software.
Here is a way you can fix this. Go to help->install new software->click on the blue hyperlink “Available software sites”
Now either import the bookmarks.xml with this as contents or do it manually.
Here you can add new links for eclipse extensions such as cdt (which can be found on their webpages):
You can now add the eclipse update links, which are:
When you’re done click ok and when you select “All Available Sites” you should now be able to install new features (such as cdt). Updates should also work (once new ones are available).