I used to have the same problem and no external tool tab in my Windows --> Preferences dialog. I believe that the best option is migrate to a newer WSAD version but until that's an option I found the following hack worked, AT LEAST FOR ME.
I copied the JDK1.2.2 tools.jar file into the $WSAD_HOME/plugins/org.eclipse.ant.core/ directory. Sorry, the JDK1.4 didn't work. I did not test other SDKs.
Then I edited manually the plugin.xml file in this same directory, and added:
< library name="tools.jar" >
< export name="*"/ >
</ library >
That did the trick.
I also found that replacing the ant jar files manually in this directory for a newer version ( ant 1.5.1 ) also worked. This may help in WSAD versions where the GUI menu isn't available.
hope this helps
santiago