I'm looking at http://developer.longtailvideo.com/trac/wiki/Player5PluginsBuilding. It mentions PluginInterface. I don't think that's right. I think it should be IPlugin.
Also, in the plugin SDK, there's a Player5Plugin.as. I'm trying to compile plugins into the player, which used to be a bit easier in JW Player 4.7. However, I did manage to get it to work. However, the plugin wouldn't actually show up until I added the following to initPlugin:
config['width'] = player.controls.display.width;
config['height'] = player.controls.display.height;
config['visible'] = true;
I don't know if that would still be the case if I weren't compiling plugins into the player.

Also on http://developer.longtailvideo.com/trac/wiki/Player5PluginsBuilding, the resize function must have type :void or else the Flex SDK barfs.