I think I almost went insane trying to figure this out!
I was trying to embed the timeline on a google sites account, and the only thing showing up was “Tweets from @Username”
So, here’s what I did:
Save that file as an xml (filename.xml)
Upload to the public folder in DropBox and copy the public link.
In the google sites page editor go to Insert -> More Gadgets -> Add Gadget by URL
Paste in the link from DropBox and Save the gadget and then save the page.
You can also upload the xml to your google sites account, and copy the download location (remove the redirect after the address when pasting though)
In Chrome, reload the page (it will still only display Tweets from @Username).
Right click the page, and select Inspect Element.
Make sure Elements is the selected Tab in the window.
Click on the little red x or yellow triangle in the bottom right corner.
One of the error messages will say something along the lines of
u2q5iu512olfq8r8lm04j7a05653tri0-a-sites-opensocial.googleusercontent.com is not whitelisted on this domain
Copy that address, and paste it into the Domains section of the widget editor on Twitter.
Save the widget, and refresh the Google sites page.
I made the mistake of just listing sites.google.com in the domains section!
Hopefully this will help some of you getting the same error (I’m assuming it’ll work to find the url that needs to be whitelisted on any site, not just Google Sites!)