Normally, admin is not a user in Ubuntu. The equivalent to admin is "root", but you don't normally need a root password.
In order to perform administrative tasks, you use your normal user account, and use a command such as "sudo" or for gui apps, "gksu".
If you do have a user named admin, then you need to log in with that name and corresponding password. But admin is just another user, not anything special.
Note that the password asked for by the updater is for the currently logged-in user, not some administrator or anything. The user must be in the "sudoers" file, or a member of the group named "sudo". In the old days, this was done by "dropping into a root shell", ie becoming root/superuser (using the "su" command). You would then enter all the administrative commands, and then type "exit" to return to your normal shell. But that required giving all users who might need it the root password, which is not good security; so, modern systems use the sudo method, which allows specified users to have higher levels of privileges without necessarily knowing the root password.