I downloaded the Eclipse Juno 4.2 classic bundle, and saw that Help > Marketplace... was not available. AFAIK, this is provided by the mpc plugin. On their website however, I didn't see a trace of how to install the mpc client for Juno.
How can I add the Marketplace Client to the classic Juno bundle?
As far as I know, the Classic distribution does not contain the Marketplace client (and that is the only such official distribution). However, it is available for download.
Select Help/Install new software... from the menu, select the Juno update site (http://download.eclipse.org/releases/juno), and then look for the Marketplace client - it is in the General Purpose Tools category.