To add plugins
Help > Software Updates . . . > Find and Install . . .
Search for new features to install > Next > New Remote Site . . .
Subclipse:
Subclipse releases for Eclipse 3.x are hosted at our update site. Add http://subclipse.tigris.org/update_1.0.x as an update site in Eclipse's update manager (which you can find in the Help menu).
The update site itself can be downloaded and used as a local update site as well. Unzip the site archive, and add the path as a new "Local Site" in the Eclipse update manager. You can also host your own internal Subclipse update site by placing the site on your own HTTP server (be sure to update site.xml appropriately). See also Subclipse's instructions for installing Subclipse in Eclipse 3.x.