With some more info gleaned from this forum and other sources, I was somewhat successful at getting most of my avi files to play properly.
I added another entry to the clients.db file for the Samsung TV. It now plays most of my avi files. There are some that still say "Unsupported codec," so the only way to get that working would be to have the DLNA server do some transcoding for those particular types of files. I haven't figured that out yet.
I copied the entry for the PS3 and modified it, giving it a new ID#. (I just moved the avi and divx options to the mpeg line). On a Mac anyway the Clients.db file is in /Applications/Mediaserver.app/MediaServer/resources. I modified it with vi(text editor) in the terminal.
Then I just selected that profile on the Clients/Security setup options in the Twonky web interface and it worked.