if you are talking about the default admin account, then you can try it this
way,
From your User Account that has Admin rights, click on Start>run> type in
control userpasswords2
Once there you find the default admin account and assign a password..but, be
very careful to not forget that password because if you do, and you need to
use that account, to fix any problems with your computer you are going to be
very unhappy.
I think you can also assign a password the default admin account from within
safe mode>user accounts, but I'm not sure.
The reason that account is not showing up in user accounts, firstly, when
you get the os installed, there is only one account, the default ( built
in ) admin. account, but once you go to User Accounts and create an account
with admin rights, the default one will not be visible unless you boot to
safe mode. There is a difference between a user account with admin rights
and the built in Administrator Account.