Sorry for the delay in replying to your post! We've been away at the FileMaker Developer's Conference all week and my time here is not really my own.
So I think this is the post you were thinking of:
http://www.seedcode.com/support/viewtopic.php?t=1631
That post suggests making a loop that would create a new appointment record for each mailing sent and describes creating a new appointment type called "letters" so you can filter the activity to show just your letters.
However, I don't know if that is how I'd do it if you're using SeedCode Complete. The Mailings sent from SeedCode Complete already show up under the mailings tab of the contact's record. So, duplicating that record in the activity (appointments) table doesn't seem to make a lot of sense. If you have to see those records beside the other activity records you might consider just moving that Mailings portal from its own tab.
I hope that helps.