[plt-scheme] removing a package

another newbie question:
I found in helpdesk this method for removing a previsouly installed pacage:

 > (remove-pkg owner name major-version minor-version) -> void
Removes the specified package from the local planet cache. Owner and
name must be strings; major-version and minor-version must be natural
numbers and major-version must be greater than zero.


However, the remove-pkg does not seem to be recognized as a command. I 
am sure someone has a very simple answer to this one, I just can't seem 
to find it...
This is what I get: reference to undefined identifier: remove-pkg

Or can I just delete it directly from (CACHE-DIR)
To make this post more useful for other newbies:
In order to use this command, you have to execute this first:
(require (lib "dirs.ss" "setup") (lib "config.ss" "planet"))
Thanks to Grant Rettke, for pointing this out to me.

Ronny