WSAD 5.1 is based on the old Eclipse 2.3. It did not have the automatical install functionality back then.
The plugins that are compatible for this version should be downloadable (most new/current plugins don't work with WSAD, as the plugin mechanism changed a little when Eclipse went to version 3). Usually they come as zip-files and
you should unzip them in the Eclipse root directory or the plugins directory (based on the structure of the zip and the installation instructions).