After registering they will provide you with an email which gives you the link you need to download the registration key. Then you use the key to register WinRAR:
The download contains a RAR archive, including your license key. Please SAVE this archive to your disk. By opening it with double click on it’s icon, WinRAR registers your version automatically and you get the information:
“Correct registration: Thank you for support”.
Hopefully this time their site will stay afloat!