[ns] what's wrong with nam?

[pyramidal]

Hi guys,
After the installation,I run a simple script exampe1a.tcl which is contained in 
ns-tutorial.However,the command window displays the followings:
 
ns: finish:couldn't excute "nam":no such file or directory
    while excuting
"exec nam out.nam &"
    (procedure "finish" line 7)
     invoked from within
"finish"
 
Where does the problem lie?
Thanks in advance!