Adding subtitles to an avi format on osx

Do you mean you already have an srt file that you want to encode into a 'hardsub', or that you want to make an srt file? Or do you just need to playback?

For encoding, you could use ffmpegX, for making you can use just TextEdit, just look at an existing srt how it's built up. For playback, just put the srt in the same folder as the avi, then open it with Quicktime with Perian installed.