Unfortunately, Google decided to stop supporting third-party widgets in the new version of Google Sites. Only Google's own apps can be added to the new Google Sites.
POWr is working on getting POWr tools back into the Google Sites Editor.
Please help us by submitting your feedback to Google via your Google Sites Editor:
Mention that you would like to see POWr integrated, or at least get the option to embed third-party widgets back.
As soon as Google Sites supports this feature again, you will be able to use POWr on the new Google Sites.