I have a number of files which are Mac AE v 4.1.1
I'm unable to open these in AE CS5.5. An error message comes up saying that I should open the files in an older version and resave, but I have no older version. Is there a work around?
As far as I know the only way to open older versions is to resave them in a newer version. This previous thread explains a lot about compatibility. You might just have to find a friend with another version and ask him to save out the files for you.
As Ben said you'll have to open the projects up in AE 5, save them, then go to I think CS4, save again, then you should be good to go for CS6 or at least CS5.5. You may be able to open directly in CS4.
Lots of things have changed but basic functionality should remain with most projects.
I use Automator (mac) to routinely update old projects, but I let the ones with little chance of being used again go.