Are you talking about the Live ATC of the JaBoG32 Dedicated Server?:
https://www.virtual-jabog32.de/atc/ATC_100.swf
If so, you can easily login by just clicking the connect button. Don't fill in any data and leave all fields as they are. Then just follow the instruction on the screen.
If you're talking about the Software itself, then just have a look into the zip package. If I recall correctly, Zillion included a brief tutorial and introduction about how to set up the Radar Server in the latest version of the ATC Client. You don't have to register at a special server and you don't need to create some sort of special account.
