AE 5.5 is built for Mac OS X.
AE 5.0 had a bunch of Carbon stubs getting ready for the eventual move to X. Apparantly Adobe also had a high-res icon build-in too. However, it will not run natively in X. It is for Mac OS 8 & 9 only. Sorry!
Time to upgrade! =)
Was it only 5.5, Parker? Wow. Anyway, just incase you can always do a get info on the application and see if it has been set to open in Classic.
yeah, it says "classic application" under get info
There ya have it!