The Server Address is icq.mirabilis.com:4000
You can register with the Shell driven micq client on Amiga, at least it worked when I did some time ago.
Or simply go to the ICQ Home Page
http://www.icq.com newlogin, register there and fill in your details.
On the Amiga there is the StrICQ client but it's buggy and unstable.
A better solution is the Jabberwocky client.
Jabber supports the own jabber protocol and AIM, ICQ, MSN, IRC through transport streams.
Public Jabberservers can be found on
www.jabbercentral.com