for my case, Eclipse indigo / juno, Ubuntu 12.04, virtual box operating system, any reinstall or update or install other version of eclipse or close/restart do not work, and toot me more than 3 hrs.... (BUT MY WINDOWS 7 HOST ECLIPSE JUNO GOT IT WHEN INSTALLED) :(
The only way I've found and worked was that, and possibly where the bug was:
For example, we want to install the eclipse marketplace.
When you choose Help -> Install new Software -> select the version of your eclipse -> GENERAL PURPOSE TOOLS, DO NOT CLICK NEXT YET, but open the list of "general purpose tools", search the sub element you want, in our case is the "Marketplace", then click Next, follow all the requirements, and when I agreed with the licences, Finish Bottom enabled immediately.
Hopefully this is where exactly the bug is, and hope similar problems can be fixed in this way. Good luck guys!