This is going to bug me all day.
I have a bunch of experience with the Morrow. Great unit. It's been a few years and I'm trying to remember how to enter those airports.
Try turning the unit on and scrolling through the different pages. There should be a "User Waypoint" page or something similar. It will allow you to enter intersections, airports, vor's, etc. Your database is most likely way out of date so some airports might not appear as the names have changed over the years.
I just looked through my bookshelf and no sign of the manual. I'll let you know if I find it.