MediaBox uses the filename extension for detecting filetypes (simply because this is the fastest way). Extension .avi is recognized. Do these files have .avi as filename extension?
Do they appear in MediaBox' file browser when browsing the local filesystem (Video Clips)?
Note: MediaBox only scan for media in the places that you specify. To add places to this library, go to the desired place with the filebrowser and press the [+] button when you are there. The "Library" tab in the filebrowser lists the folders that are selected for scanning. There you can specify for what media types (video, image, and audio) MediaBox should look for. By default, it scans for all three of them.