In case if you have lost agent registration pwd and want to create new one. Go to setup=> registration pwd and remove the existing one.
then in the OMS server execute below command,
/emctl secure setpwd
Oracle Enterprise Manager 11g Release 1 Grid Control
Copyright (c) 1996, 2010 Oracle Corporation. All rights reserved.
Enter Enterprise Manager Root Password :
Enter New Agent Registration Password :
Registration Password added successfully.
root password above is sysman password