Switch back to your document and reload the page.
After a few seconds, a Translate Quickstart sub-menu will appear under the Add-ons menu. (If you chose a different name for your script, that name will appear instead.) Click Add-ons > Translate Quickstart > Start.
A dialog box will appear and tell you that the script requires authorization. Click Continue. A second dialog box will then request authorization for specific Google services. Read the notice carefully, then click Accept.
A sidebar will appear. To test it, type some text into your document, then select it and click the blue Translate button. To replace the text in the document, click Insert.