.avc files

One method is to download and use VLC Media Player to watch or convert them. Another method is to convert them to Quicktime format using a video converter app and watch them in Quicktime which is pre-installed on the Mac.

VLC converts to Quicktime and if you get the VidConverter app it does the same. I don't know how to be more clear than that. The .avc file format is a MS Windows formate, in order for a Mac app to open a .avc file it must first be converted to a file format that a Mac can understand.