I don't recall having a problem installing the Scala plugin on my old (Windows) laptop, but I got a new one and started migrating things over this week.  In order to get the Scala plugin to install, I had to install the Eclipse plugin development environment. I was happy to have found this blog post, which explained what to do to get around the error ("org.eclipse.pde.runtime is missing").  So I guess I must have either had 3.2 installed on that machine (it's been upgraded to 3.3 now), or I had a more complete install. All's well now, whew!