I recommend renaming the one you've created at installation time, but I don't know the side effects.
First, log off from the web interface!
Try this in a terminal (make a backup first!):
Check your real user id:
id
If it's really administrator, try to rename it to "myadmin":
sudo usermod -d /home/myadmin -l myadmin administrator
sudo mv /home/administrator /home/myadmin
Reboot your system immediately!
After reboot, you should be able to log in on the console and to the web interface with the new name and the old password.
I hope that the Zentyal's "Administrator" account (visible in the users list) is there and you can use it. Change the password for that user!
I hope it helps, but be aware that you can lock out yourself from your system, so make a backup!
Regards,
Krisztián