Yes I have Eclipse Java EE IDE.
Eclipse Version: Kepler Service Release 2
First I downloaded 'Eclipse Plugin for ThingWorx Extensions zip' file from ThingWorx marketplace.
After downloading this extension I am following below steps:
1. Selecting 'Install New Software' option under Help menu, Install screen appear.
2. Clicking on Add and added downloaded 'Eclipse Plugin for ThingWorx Extensions zip' file.
3. Selecting ThingWorx Extension Builder after deselecting the 'Group items by category' checkbox and Click NEXT...
4. After clicking Next it takes few minutes and throw the above mentioned error.