If it has a barcode on it I'd assume it's something you'll trade for a proper ticket when they scan it, that's what they did for Metallica few years back. As you didn't buy it through nin.com presale I doubt you'll need ID, but check the terms and conditions on the website to be sure. Entrance F is an entrance to the standing area itself, not the arena. There's only one entrance to the arena, so just follow the black t-shirts! F is quite a walk from the main entrance, as per the map below.
http://www.theo2.co.uk/images/PDF/mi..._venue_map.pdf
You won''t be able to enter the arena itself until 18.30, so 5pm is more than enough time to get there, get settled and get a couple of pre show beers in....I should be there around 5 - I'll probably just go to the American Bar and Grill to start (unless anyone has any better ideas within the arena space), 1st bar on your left as you walk in - probably wearing a LITS US tour shirt with a red NIN on the front - so come over and say hi if you see me.