Two solutions exist for this problem in the install session:
1) Retry the agent installation using the correct password
2) If the password is unknown or it is desired to change it, access Grid Control's Setup link and then the Registration Password link on the left of the resulting page, and use the Edit option in order to change the securing password, then retry the agent installation with the one entered.
Note that the second option may result in access problems with existing and properly secured agents.
It is also possible to unlock the OMS, verify the agent is installed and uploading properly via http and then secure at a later time to allow uploading via https.