Just uploaded Motion 5 on my new 13" Macbook Air, and it reports "Unsupported Format" as I try to use elements created in Avid MC. I have the very latest Avid version (7) loaded, and for good measure loaded in the legacy codecs as well. Restarted. No joy. Any thoughts? Thank heaven for the Creative Cow community!
In my opinion Apple doesn't play nice with the other kids in the sandbox and doesn't support as many codecs as Adobe does. I would recommend exporting out your files as a Quicktime movie like ProRes. It supports basically any Quicktime codec installed on your computer. Here is a link of supported files if that helps: http://help.apple.com/motion/mac/5.0/en/motion/usermanual/index.html#chapte... Best of luck.
I use Motion 4 on my older 10.5.8 intel Mac, and it allows me to utilize Avid assets without fuss. In fact, the problematic project was exported from Motion 4 as a test. I'm thinking the Avid uncompressed codec is not being seen by Motion 5.