To get on the US site from the UK you need to log on to the UK site, then from the home page go to the bottom left hand side of the page and you will see some flags and a drop down box, choose USA English (you may need to do this twice) and let the page refresh. When the page refreshes in the top left hand corner (under where it says Parks/Resorts) it should say "visit US site", click here and voila you are on the US site. Then all you need to do is choose the book tab and click on dining reservations.
As for making ADR's I would say DO IT! we have just returned and had 14 meals that had been booked and we had no problems, the longest we waited for a table was 20 mins at Teppan Edo and that was because we arrived early for our ADR, my theory is that you can always cancel ressies but if you want premium meals Le Cellier, Ohana's, CRT etc then you need to get that ressie at 90 days (180 from OCT).
Hope this helps, good luck