Adding a Word Index to your transcript is as easy as point and click.
With your transcript open, navigate to the last page and add a blank page by hitting the enter key on your keyboard until the blank page appears.

With your cursor placed at the beginning of line number 1 on the blank page, select the "Insert" tab, and click on "Word Index".
And your Word Index will populate the page.
NOTE - Longer transcripts may take some time to produce the Word Index