Pottermore was slated to be available to the public this past October. But in its beta version that was open to only a million users, they received valuable feedback alerting them to the fact that the original platform they planned to use just wouldn't handle the anticipated volume of users. So they made the decision to move Pottermore to a completely different platform set up.
They still plan to have the entire site up and running in early April. Fingers crossed.
The Pottermore folks have uploaded this handy video tutorial to YouTube: