问题解决了。。。
http://www.easywms.com/easywms/?q=en/node/97
1. Delete the site.xml file in your Eclipse directory.
2. Run Help->Software Updates->Find and install, the error will not appear.
If the plugin has the official site, from where you can install it in Eclipse using Help->Software Updates->Find and install, I sugguest that you should not download it yourself and unzip it into your Eclipse directory.