Try following these directions. While they're slightly old, they should still hold: http://alleg.sourceforge.net/onlinedocs/en/build/macosx.html. There are instructions for building a template that will create a default XCode project that should compile right off the bat.

One thing to note: the last time I did a build using this setup the template it created catered to an older version of XCode. That means that if you have the latest version, even after creating the default Allegro application this template process provides, you'll have to upgrade the actual project to native. There's an option in the Project menu in XCode to do so.

It's worse than that - it's a Project Builder template! Someone probably needs to update that, but I still haven't been willing to install Allegro on my new Intel Mac because it insists on dumping stuff into /usr and other such UNIX places that I'll very quickly forget about. Which is not particularly desirable for me personally.

Off topic slightly, but does anybody have, or know of a link to, an already built univeral binary Allegro framework?

If Xcode couldn't find your header file then it wouldn't be able to compile. If your program exits with sigbus error 10 then it is crashing so something is probably at fault with your code, but possibly at fault with Allegro. Can you post the code you're using?