In order to sign up for a Monthly Credit Bundle, you need to first log into your account. Click on the "Manage Subscriptions" button, then choose the Monthly Credit Bundle.
Click on the "Subscribe" button and your done! Your credit card will be charged the $49 when you sign up for this and 65 credits will be added to your account instantly. This will happen each month usually on the same day. You'll receive the credits and be charged for them as well.