There should have been two emails from Jon -- did you read both?
MaG's walkthrough states:The first [email] explains on how to translate the Ogam scripts. If you want to do it manually, use the 2 attachments at top right of the mail as guide. The first attachment is to translate the scripts to alphabets. The second is a dictionary of the 3 letter groupings.
The second mail is his Ogam Translator Program. Let's use this one.
Click on the blue Ogam Translator Program.
So it's the second email that has what you need. That should then resemble the screenshot in the walkthrough and you can proceed.