You need to provide much more info about your system, the file and so on. AVI is jsut a container format that can contain a multitude of different audio and video CoDecs, some of which are not compatible with AE. Chances are you will have to convert it externally.
After Effects CS6 does import many different AVI files. And there are some it won't. As Mylenium says, AVI is merely a container holding a codec. What codec your footage uses will be a major factor in how (or if) it gets imported. Please provide the info he requests.