You're a member of the forum but do you have access to the special forum? I assume that was the forum you were referring to. And if I'm reading correctly, you're wanting to sign up for the Alpha? If you haven't already signed up (early this month or before) then the applications are now closed. If you did sign up and you received an email saying you were selected, then create a new forum account with the email you used for the Alpha Application.
Special Forum For Alpha Testers: https://forums.focus-home.com/category/208/insurgency-sandstorm-closed-technical-alpha