No unfortunately there isn't an AMV converter for the Mac.
You have a few options though. Buy a cheapo used Windows PC or borrow a friends for the conversion (obviously).
You could use
http://media-convert.com/convert/index.php to convert your videos to AMV... but it takes an eterinty to upload and convert your videos here... painful. And the conversion resolutions are so small it's not worth the trouble.
Another option would be to see if theres a free Windows emulator for your Mac.