Follow these steps to access the Warframe Closed Beta:
1. Go to www.warframe.com/signup to Create an Account or to Finalize your Account if you reserved on Warframe.com previously (note: please use the exact same email address and alias you reserved earlier)
2. Check your email! Warframe will send you an email with a link that will authenticate your account and bring you to the BETA KEY ACTIVATION PAGE (note: check your junk folder too).