How to recreate an Admin Account on a MAC:

hold CMD-S or WIN-S and power on the MAC.

now some manual editing:

fsck -y

mount -uaw /

rm /var/db/.AppleSetupDone

reboot

The System now thinks it is not even Set Up yet. Now you will be creating a new User and go through the Setup process. This user becomes an Administrator on the Mac after the setup.

Have fun.

After writing this i saw the Article from Dennis