How to use TwoStateMarkov error model?

["Seo, Chiduk" <suchduck@netdelta.sogang.ac.kr>]

ns-errmodel.tcl reads that a 2-state Markov error model is built-in,as
ErrorModel/MultiState/TwoStateMarkov and I have seen several scripts
using ErrorModel/MultiState/TwoStateMarkov in this mail archive.
But, I have failed. 

my script is shown below.

------------
set     unit    pkt
set     trans   [list 0.001 0.999]
set     rate    [list 10 1]

set     em      [new    ErrorModel/MultiState/TwoStateMarkov $rate
$trans $unit]
------------

the result is shown below.
--------------
invalid command name "ErrorModel/MultiState/TwoStateMarkov"
    while executing
"ErrorModel/MultiState/TwoStateMarkov create _o36 {10 1} {0.001 0.999}
pkt"
    invoked from within
"catch "$className create $o $args" msg"
    (procedure "new" line 3)
    invoked from within
"new    ErrorModel/MultiState/TwoStateMarkov \
$rate $trans $unit"
----------------
ns version is ns-2.1b3.
why invalid command name "ErrorModel/MultiState/TwoStateMarkov ?
if you know what the problem is, please help me. 
I wanna use TwoStateMarkov as error model.

Thanks for your reading.

suchduck@netdelta.sogang.ac.kr