I don't think you can actually change a Sim's ears. The only way to get faerie/elf ears is to use the face template with those ears, or to apply accessory elf ears... 
Ren at MTS has some nice elf ears: 
http://www.modthesims.info/download.php?t=216324 There are also some recolors to match different custom skin tones, but I can't remember where I found those. Probably on MTS as well. 
Hope that helps!  
