[For information on rendering and exporting, see "Basics of rendering and exporting".]
On Windows versions of After Effects, AVI appears listed with the more comprehensive name, "Video for Windows". In fact, it's the format specified by the default Output Module template. The Format Options button will reveal the AVI/VfW codecs available on your system.
On Mac systems, AVI/Video for Windows is not available as a direct export option (except through the File > Export path, which is really not recommended). Export a lossless intermediate file from the Render Queue instead (for example, a Quicktime file). You can then use a number of converter applications (including Quicktime Pro, for example) to convert this intermediate file to an AVI file.