Note that checking Enable Templated Email is not necessary to add custom text to the product's thank-you email message. If you leave it unchecked, whatever you enter in the Email Message field would be added to our standard greeting and link presentation for download items.
If you do checkmark it, whatever you enter in the Email Message field would REPLACE our standard greeting and link, so the custom Email Message you provide should include the template tag [%download_link%] (ideally on a line by itself) wherever you want us to insert a link to the buyer's download page.