When I got near the arena grandmaster there was a yellow thing which said missing meshes I don't know what causes this. I use FCOM.

while in game and in front of the yellow sign open the console (the ` button usually) and then click on the yellow missing mesh.

write down that number.

Then open your mods in tes4edit and put that number into the search function and it will tell you what is missing.

You could also install http://www.tesnexus.com/downloads/file.php?id=16704 and it will tell you in game.

Likely you added something that changed the mesh path to a custom mesh and then forgot to add that mesh.

Thanks.