"Codec" is not the same as "container".

"codec" examples:

mpeg-1, mpeg-2, mpeg-4 (divx), indeo5, uncompressed video (not a codec?)...

 

"container" examples:

mpg, avi, asf... 

 

AVI can even contain mpeg-1 video.

I don't know what kind of video poser supports but you can download some free program like virtualdub or gspot to get information what codes is used in your avi.

 

Then find video which works with poser and check which codec was used here (so you can use the same one when writing avi in bryce)