thanks a lot man..my problem is solved..the problem was bcoz the plugin was not properly installed in /usr/local/nagios/libexec
..now i am facing few more problems
1) When a host is down i am not getting alert.we are using exchange.
2) i am getting a Error: Could not open command file '/usr/local/nagios/var/rw/nagios.cmd.i did according to the steps which is mentioned below
chown nagios.nagios /usr/local/nagios/var/rw
chmod u+rwx /usr/local/nagios/var/rw
chmod g+rwx /usr/local/nagios/var/rw
chmod g+s /usr/local/nagios/var/rw
i have logged in a root and there is one more user called nagios.even if i delete the nagios.cmd file its again created with the same permission.
3) In /usr/local/nagios/var/nagios.log i am getting a error msg which s mentioned below
 SERVICE NOTIFICATION: nagiosadmin;dev-sudhesh;Memory Usage;CRITICAL;notify-service-by-email;Connection refused
 SERVICE NOTIFICATION: nagiosadmin;dev-sudhesh;C:\ Drive Space;CRITICAL;notify-service-by-email;Connection refused
 Warning: Return code of 127 for check of service 'Printer Status' on host 'hplj2605dn' was out of bounds. Make sure the plugin you're trying to run actually exists.
 HOST NOTIFICATION: nagiosadmin;hplj2605dn;DOWN;notify-host-by-email;(Host Check Timed Out)