It looks like with the latest release, the admin script no longer function ("Cannot connect to vSphere Web Client administration tool.") and since the VCSA does not contain a full blown desktop with a browser like it's ugly Windows cousin ... you will not be able to register any of your existing vCenter Server 5.0 systems. Luckily, you do not need a browser running on the VCSA to perform the registration, you can use the browser on your desktop by simply using SSH port forwarding.
In the example below, I am connecting to my VCSA 5.1 from my iMac via terminal. You will need a system that can reach your VCSA that has a web browser which will be used to access the Admin Tool.
Step 1 - SSH to your VCSA using the following command, be sure to replace "172.30.0.194" with the IP Address or hostname of your VCSA
ssh email@example.com -L 9443:127.0.0.1:9443 -NIf the port forward was established correctly, the prompt will just sit there as denoted by the screenshot below.
Step 2 - Open a web browser on your local desktop and connect to the following address:
https://localhost:9443/admin-appThis should launch the vSphere Web Client Admin Tool and allow you to register your vCenter Server 5.0 hosts.