it is important not to panic. Very few people know the answer offhand,and nobody knows exactly when it will open. The best you can do is:
Between the hours estimated by the official Potter blog (2and 6pm bst,ie between 5 mins from now and 4 hours thereafter);
Refresh the POTTERMORE.COM page every few minutes. It seems boring,but only good luck will make you check it at the exact time.
Keep your Goblet of Fire handy...and lets not forget a calculator as the clue is always a number to multiply to another.
Finally when you work out the number, add it to the url http://quill.pottermore.com/ eg. On day one i was lucky enough to catch it on http://quill.pottermore.com/258 .
You will be brought to a very specific page of a harry potter realted website, day 1 was sony Pottermore’s partner, day 2 was warner bros photos..day 3 i wasn’t on for.
LOOK FOR THE MAGIC QUILL, WHICH WILL BE LIGHTING UP. CLICK ON THAT, FOLLOW THE LINK IN THE CONGRATULATIONS AND REGISTER ALL YOUR DETAILS, DATE OF BIRTH, NAME, HAVE YOU READ THE BOOKS AND COUNTRY...you can then choose a silly username.
Be patient yet again as emails aren’t always quick at getting to you. When your validation email comes,follow the link, sign in and you should get the message to the left.
BUT MOST IMPORTANTLY OF ALL, there are still 3 days left if you don’t get in,and even if you don’t, its NOT THE END OF THE WORLD, It will be open in October anyway, for all, fully developed,free of charge.