Download the .xml file linked on the page below.
Right click on the file, choose "open with" from the menu, and select Notepad.
Go to Twitter settings, select "Widgets", create a new widget, and then copy the embed code that's shown for the widget.
Paste the embed code into the designated place in the .xml file that you've opened in Notepad and save.
Go to your Google Site, open the settings, and select "Manage Site". From there, select attachments and upload the newly saved .xml file.
Right click on the download button and select "copy URL". Paste the URL somewhere and remove all of the text that follows the .xml extension.
Go to the place you want to embed your Twitter URL and select "... more Gadgets...". From there, select "Add Gadget by URL" and paste in the URL of the .xml file that you've added.
Tweak the settings of the widget to fit your needs and start sending out those Tweets!