How many smiles are in the XML file. I found that I had to make a php.ini change to use 2 of the ones that Sheldon did (the emoji extended and one of the other ones). I also got an error when I was importing them (did not show in the server error log) but had to actually check my nginx and php error logs.
The actual error I got was (not in Xenforo but in my server logs)
PHP message: PHP Warning: Unknown: Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini
This also caused problems with the drag/drop ability in the ACP smilie manager.