The activation process involves linking your computer to your license.
If you lost your key, contact u-he support. They will help you. If you cannot afford Diva, use the free alternatives or the unlimited 30-minute demo mode. And when you can, buy Diva. It is worth every penny. Not only do you get a world-class synth, but you also support a company that treats its customers with respect—no invasive DRM, no iLok dongle, just honest software for honest musicians.