It wasn't quite as easy as I would have liked to get the MercurialEclipse plugin working, but it certainly could have been worse.
It was easy to install the Eclipse plugin using the update site: http://cbes.javaforge.com/update
But when I tried to import my project from Mercurial I got a bunch of errors about passwords.
I found a post from someone with the same problem, with a pointer to instructions on how to solve it.
I guess the simple username + password doesn't work with MercurialEclipse, at least on OS X. Sigh.
The answer is to use an SSH key, which I'd avoided till now, but it turned out to be straightforward with the help of the SourceForge instructions. It sounds like it might be a bit trickier on my Windows box - I haven't tried that yet.
I tweaked a few things for the new setup, committed those changes, and pushed the changes to the SourceForge repository. Seems like I'm good to go :-)