Never mind. I figured it out. It's not intuitive but it is easy.
You have to enter your MileagePlan number and then request a PIN for points.com to use to access your account. You do this by clicking on the "Obtain your PIN" link next to the PIN field. As far as I can tell the only purpose for this PIN is for points.com. You would never need to use it on AlaskaAir.com