drprogres: Mocha AE is developed by a third party company (Imagineer Systems), so their importer technology is not identical to AE's.
From what I can see, Mocha uses Quicktime to load AVI files. This means that you can open AVI files as far as they use the standard set of AVI codecs supported by Quicktime.
You could convert your source file to a number of production formats, including image sequences and standard Quicktime/AVI codecs.
In case you're interested, there's a very active forum on Mocha, hosted by Imagineer themselves.