DoesNotUnderstandError

If you get a message like

ERROR: Message 'kr' not understood. RECEIVER: nil

(or it could be 'ar' or 'ir' instead of 'kr')

then you're probably trying to use a UGen that isn't installed in your class library. Remember that when you install a UGen you need the plugin for the server (file extension .scx or .so) PLUS the class file for the language (file extension .sc).

For more info see the helpfile "Using Extensions".