I'm running Eclipse 3.6.1 Classic, which does not come with the Eclipse Marketplace plugin by default. I've looked around the Eclipse website, but I don't see an available plugin for installing Eclipse Marketplace. Am I just not seeing it?
Help → Install new Software → Switch to the Kepler Repository → General Purpose Tools → Marketplace Client
If you use Eclipse Luna SR 1, the released Marketplace contains a bug; you have to install it from the Marketplace update site. This is fixed again in Luna SR 2.
Marketplace update site: