i see from thread title you have Max 2011. Max 3 is simply an earlier version of max program, unfortunately no longer commercially available...to best of my knowledge, though you might be lucky enough to find it on ebay, etc. Others can correct me if wrong, but Max version 3 was last version of the program that allowed exporting to BC ( I assume as they were around about the same time). There was a special BC exporter created for Max 3 which you need to have to export to nif. It was never updated/rewritten for later versions of max. Hope that helps?
Bottom line: your 2011 import nif will work with BC models and allow you to tweak, fiddle, etc; the 2011 export nif is pretty much defunct for BC. You have to get copy of Max 3 if you want to export back into BC.