In the latest patch, a minor mistake was made that did not allow for the custom battle models of two major characters involved in the Liu Bei missions.
(If you don't want any spoilers for the Liu Bei missions, do not open the spoiler)
I have made a hotfix to correct this. Download the attachment in this post and extract to your Medieval II Total War/mods/Three_Kingdoms folder.
It is NOT SAVEGAME COMPATIBLE, so you must start a new game for it to take effect. This will be included in Patch #5, so whenever that patch is released this will become obsolete.
EDIT: New Hotfix by Gigantus includes this. Upload taken down