The radio button can be used to set up the appropriate next step. If you have used ICQ before, then switch the button over to "Existing UIN" (a UIN is a "Universal Internet Number", which is how the server and other users tell you apart from other folk on ICQ). If not, then leave it as you see it as "New UIN".
This is the panel you will next encounter. In this case you are a new user, and ICQ knows nothing about you. Your mission, should you choose to accept it, is to fill this form in with as much information as you feel happy with sending. Note that the password is mandatory, as is the email. Remember your password since if you ever need to reinstall (say, for example you have another computer that you wish to have ICQ on) you will need this password to connect with. Click on Continue once everything has been input.
Note that the "All users may add me" and "My authorisation is required" options are on mutually exclusive radio buttons. If you leave the button as "All users" then anyone can add you to their list (although you will be told when they do -Not in Acorn ICQ version 0.10-). Otherwise you will have to give your permission before any person can add you to their list.
On the other hand, this is what you get if you are an existing user. Enter your UIN into the UIN box, then enter your password into BOTH the password boxes. You will then connect to ICQ and the server, if it remembers you and you got everything right, will fill the rest in for you. Click on Continue to carry on.