How to use 'svn diff' command

I am trying the example here (the first example) to diff between my
working copy and the trunk.

http://svnbook.red-bean.com/en/1.1/re09.html

But I get this error:
$ svn diff COMMITTERS
svn: 'COMMITTERS' is not under version control

I try this too, and it sill does not work:
$svn diff TRUNK
svn: 'TRUNK' is not under version control

Please help me understand how to use teh 'svn diff' command. I don't
know where does the argument ''COMMITTERS' come from when I try to
compare the BASE of my working copy.

From the example page:
Compare BASE and your working copy (one of the most popular uses of svn diff):

$ svn diff COMMITTERS
Index: COMMITTERS
===================================================================
--- COMMITTERS (revision 4404)
+++ COMMITTERS (working copy)

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Received on Fri Jan 19 17:35:10 2007