The SDK Installer requires that you enter an activation key. After completion of the sign up process you will have received an activation key in your email.
Your activation key should not be shared with any one else as it is unique to you. If you are developing your project with other users, they must sign up for their own activation key which they can enter in the SDK Installer window.
If you don't yet have an activation key, you can get one for free by signing up here.
Updated over 2 years ago