The admin account is ROOT and is disabled by default kinda. Actually, the password is not set and the system is configured not to allow blank passwords. You can change the admin password, it is in the Wiki.
However, log in as PI and it should suffice for 98% of things. If you need to run a command as an elevated user, prefix the command with sudo.
“sudo YOUR_COMMAND_HERE”
See This page for other info: https://github.com/retropie/retropie-setup/wiki/FAQ
Why can’t I SSH as root anymore?
Starting with RetroPie 3.0 BETA 3 the root password was disabled (as is the case for Raspbian by default and many other linux distros).
If you would like to re-enable root access, in the terminal type:
sudo passwd root
see these posts for more details:
https://www.raspberrypi.org/documentation/linux/usage/root.md
http://elinux.org/R-Pi_Troubleshooting#I_don.27t_know_the_root_password
RetroPie 3.0 Jessie Builds:
before setting a root password, the following must be edited
sudo nano /etc/ssh/sshd_config
look for
PermitRootLogin without-password
change it to
PermitRootLogin yes
then ctrl+x to save, next set your root password & restart your Pi