first want you need is:
Quicktime
3ivx- d4 4.5.1
Divx Bundle- 6.0 pro
Ac3filter- 1.0.2
you will play .avi in quicktime.. 
How to play .avi files in quicktime
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Yup, you need to install right CODEC first.
http://www.free-codecs.com/download/Codec_Pack_All_in_1.htm
Or, convert your AVI video to QuickTime video with Mac video converter.