I installed the Eclipse Marketplace via http://download.eclipse.org/mpc/indigo/ and Help>Install New Software... and it's not showing up under the help menu as expected after a complete reboot and I can't seem to find anyone else who is having this problem?
What led me to installing the Eclipse Marketplace in the first place was my attempt to install Eclipse Color Themes (http://eclipse-color-theme.github.com/update/). Eclipse Color Themes did not appear where indicated in it's documentation either so I thought I would attempt installing through the marketplace but I think I may have a larger plug-in related issue however plug-ins like Aptana and other misc. custom plug-ins installed correctly.
I'm running Eclipse on Windows 7 64-bit if that's of any help as well.