Fixed this by installing "Mercurial Eclipse by MercurialEclipse Project, EPL" as listed in the EclipseMarketplace.
I had "MercurialEclipse (was: HgEclipse) by Intland Software, EPL" installed, which was a version from before HgEclipse rebranded to MercurialEclipse. Even though I had installed using the instruction found at http://javaforge.com/project/HGE#download to Install New Software: http://cbes.javaforge.com/update, it seems that didn't install/update properly. When I used the EclipseMarketplace and installed the "Mercurial Eclipse by MercurialEclipse Project, EPL" plugin version, it claimed I already had it installed, but updated anyway, and that fixed the problem.