Admin and password have be changed on the ACIIIPro unit itself. You can't change this on the web iterface for security reasons. See page 14 of your manual for the menu tree inside the ACIIIPro and you will see the login and password accessible under net setup. There is also a brief description of setting this up on page 20 of the manual.
You have to do extra work to access the AC from the internet. This is network admin work you will have to do to configure your home network so that access will be allowed from the internet to your ACIII. Most people have a home DSL/Cable router that their computers are attached to. In addition, most people have dynamix ip addresses instead of fixed ip addresses (mine are fixed so it was a little esier for me).
Your home router implements a protocol called Network Address Translation (NAT for short). This means that your public ip address that your isp asssigns you (and that is accessible from the internet) is 'translated" to your private ip addresses that exist on your home network behind your router and firewall. This is done to keep hackers out of your computers that exist behind your router/firewall.
Specifically, when a you browse a web page and request info, the info is sent back from the internet to your public ip address. Your router then receives this information and "routes" it to the specific computer on your home lan that requested the info. In doing this, it routes the info to the private ip address that exists for that computer behind the router/firewall. Usually something like 192.1.0.*
So, you need to create an exception on your home router to allow web page requests (to your ACIIIPro) from the internet to be routed directly to the ACIIIPro. You are effectively creating a tunnel through your firewall and home lan for these requests to go specifically to your ACIIIPro. To do this, most routers have a setup feature. On my linksys it is under applications and gaming. From there you go to port forwarding. You want to forward all requests from the internet for web access to your fixed ip address on the ACIIIPro. The specific port you need is port 80. Port 80 is the port for web requests. So, you would forward all requests from the internet beginning and ending on port 80 to the fixed ip address for your ACIIIPro.
Now, the next step is how to get to your home router from the internet so that these web requests have a chance to get to your ACIIIPro. In order to describe this, it depends on whether you have fixed IP addresses or dynamic ip addresses. If dynamic, I will have to describe more details. Let me know on this. Have to run some errands for a bit, but will be back.