How do I activate the free version of Genymotion Desktop?

If you mean the Genymotion SaaS free trial, click Request Trial in the top banner after your account is validated. Then choose Individual or Entreprise and submit the form.

For Individual, approval takes up to 5 business days and includes 60 free minutes with 2 simultaneous devices. For Entreprise, approval takes up to 2 business days and includes 1000 free minutes with up to 10 simultaneous devices. See How do I get the free trial for Genymotion SaaS Cloud? and Get Started with Genymotion Desktop.