thanks for letting me know. You can, however, perfectly use your 
custom action only inside the mapping table for iTunes (which is what 
I'd recommend seeing you only want this functionality in iTunes 
anyway) instead of using it in the global mapping table.

To do that:

1) Reset your global mapping for the Apple Remote to revert you change.

2) Go to Mapping > iTunes and assign your action to the button of 
your choice with one more click.

Best regards, 
Felix