Monday, 28th March 2011, 2244hrs jw Eclipse
After you've downloaded and installed Eclipse Classic (like the Helios release, or version 3.7), the Marketplace client plugin will not be installed.
You can however find and install this plugin via the Help -> Install new software...
menu. Check the Helios repository and search for Marketplace:
After successful install you'll be able to install plugins using the market place via Help -> Eclipse Marketplace...
Enjoy!