Which externals have you tried? If you haven't already, I'd recommend trying some of the more commonly used externals (maybe the Tristan Jehan objects, or Tap.Tools) and see if those load. If they do, then your problem may be more with a specific external.
BTW: The thing that Mathieu just posted: I just went and fixed it for PM.Chamberlin~, so you might download it again and see if that fixes it.
this question might look better on the dev forum, but you're correct, Force Package Info Generation should be set to 'Yes'. If you use the maxmspsdk.xcconfig as a basic settings file (which is submitted with the Max SDK), then you'll find the line 'GENERATE_PKGINFO_FILE = YES', which is the reason why you don't have to set it manually in your externals (again, this is the case only if you use the mentioned file as a base for your XCode settings). It can happen, by chance, that you end up having a pkginfo in your external without having this option enabled, but that is, as I mentioned, pure chance (for instance, if you had a pkginfo in a generated external for some obscure reason and later you uncheck the 'Force Package Info Generation', then you won't lose the already generated pkginfo file).