As others have already noted here, this is a permission issue. Try changing the ownership of the eclipse installation to chown :
$ sudo chown -R root:$(id -g -n $(whoami)) /path/for/eclipse/stuff/
where the expression $(id -g -n $(whoami)) returns your main group. From there, I hope you will have permissions for everything you need. It worked for me, but I'm not an expert.
EDIT :
Another possible reason for this is creating a workspace on another computer or installing Eclipse. Different computers and / or different Eclipse installations may use different versions of plugins and JREs, which leads to all problems.
However, even if your workspace was not initialized with another Eclipse installation, try installing the plugin in a new temporary workspace.
And for the record, I found running Eclipse as root ( sudo eclipse ) as a terrible idea. Almost nothing works - from downloading to installing new plugins - when I tried it.
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