Unfortunately, that is the one type of app that does not really exist for mac. There is Route 66, but it is a horrible app -- hard to use, ugly, etc. I have been searching for an app like DeLorme Street Atlas, but to no avail.
Having said that, if you are using GPS coordinates, etc., you might try Route Buddy. Not the same as the DeLorme, but can find coordinates.
Good luck! (Actually, I am hoping somebody will post some updated info on a new app - but not holding my breath!)