This plugin is in alpha stage, at this moment. May be unstable.
Latest webtools JSDT
Latest kepler release (should work on juno too but I'm not sure for now.
Add update site into your eclipse kepler instalation
http://extjs.w3des.net/p2/ - stable http://extjs.w3des.net/p2-dev/ - snapshots
Attention: WTP 3.5.1 was released with some BC breaks and bugs. If you want using it with this plugin, p2-dev repo will be required!
Please read this article