Modern versions of Nexus use a cloud-based license or an iLok. Ensure you are connected to the internet for the initial activation.
Downloading cracked VSTs is not only illegal, but it’s also dangerous for your music computer. "Cracked" installers are notorious for containing malware, ransomware, and keyloggers that can steal your passwords or corrupt your project files. Furthermore, these versions often crash DAWs and are impossible to update.
ReFX has released Nexus 4, and it is fully backward compatible. This means If you buy the latest version, you get the modern interface, high-res graphics, and all the classic sounds you are looking for, but with better stability and support.