I'm using Eclipse for several projects on Leopard OSX. I've installed several versions (Classic, PDT, etc). When I install a new plug-in using the Help > Install New Software functionality and I restart it, I always get the same error and I cannot anymore use it:

!ENTRY org.eclipse.equinox.launcher 4 0 2010-04-20 17:32:42.540
!MESSAGE Exception launching the Eclipse Platform:
!STACK
java.lang.ClassNotFoundException: org.eclipse.core.runtime.adaptor.EclipseStarter
    at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:317)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
    at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:556)
    at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
    at org.eclipse.equinox.launcher.Main.run(Main.java:1311)

What is causing the error? How can I get these plugins to work?

Thanks