What is your Eclipse version? Java EE 4.2.2 or 4.3?
I've downloaded and installed on both my Windows 7 and Mac OS X 10.8 with Eclipse Java EE 4.2.2 for Developers (the currently supported version together with Eclipse Classic 4.2.2).
- Make sure that the option to "contact all update sites during install to find required software" is checked (in Help >> Install New Software...).