When I register for Unity, why does the internet activation, and the manual activation fail?
That's like asking "Why is my internet down?"... there are theoretically hundreds of possibilities, and since you provided as little detail as possible in your question, neglecting to include an error message or any sort of explanation as to what's going on (specifically), we can't help you at all.
Besides, for activation issues, you'd be better off contacting UT at support@unity3d.com.