tta-sandia,
Eclipse Marketplace Client is available in all packages of Eclipse except Eclipse Classic. Since MyEclipse is based on MyEclipse Classic, that feature is not available.
Please refer to the following link for further reference –
http://marketplace.eclipse.org/marketplace-client-intro
Most of the plugins that are compatible with Eclipse are compatible with MyEclipse. You can search for the required Eclipse Plugin and add it to MyEclipse. But please make sure that the pulgin is compatible with the base version of Eclipse(3.5.2 for MyEclipse 8.5 and 3.4.2 for MyEclipse 7.5).
Another related issue: how can I install a plugin just using the plugin’s jar file? E.g., a plugin which provides me a download, but no site.
If you find a .zip file of the plugin, you can extract it and drop the /plugins /features of the plugin to MyEclipse dropins folder (~\MyEclipse 8.5\dropins)
Let me know if that helps