I tested some TS3 Default Replacement Eyes in The Sims Medieval Game and I thought that I will make a list of files that works fine in The Sims Medieval so you don't need to test them yourself. I hope this will be helpful.
Remember that you can have only one Default Replacements in your game, so choose one from the list below.
http://linna.modthesims.info/d/372768 by aikea guinea
http://linna.modthesims.info/download.php?t=348047 by aikea guinea
http://linna.modthesims.info/download.php?t=437504 by ne14gzus93
http://linna.modthesims.info/download.php?t=437723 by shadowwolf5889
http://linna.modthesims.info/download.php?t=431387 by Robodl95
http://thesimsfieldofwm.blogspot.com/2010/02/eye-eyebrows-blush.html by subaxi
You must put the file into:
My computer/C:/Program Files/Electronic Arts/The Sims Medieval/Mods/Packages
You must put Resource.cfg file into:
My computer/C:/Program Files/Electronic Arts/The Sims Medieval
Resource.cfg file is attached to the first post here http://www.modthesims.info/showthread.php?t=437510&c=1&ht=&page=1&pp=25#startcomments you can find some game mods in this topic as well
I will be updating this post as soon as I test some more eyes ;) Greetings