The game works for me in Win8 in both IE Metro and desktop browsers.
I have also downloaded chrome and Firefox and works in them as well.
Best tip is from the main metro interface, click the desktop to get to the old windows 7 style interface and then launch IE and go to thesettlersonline.com you should then have a working game.
If this fails or you are running windows RT (for instance on the MS Surface tablet) then you may need to amend the IE Whitelist as per the link below.
http://forum.xda-developers.com/show....php?t=1961793
Please post and confirm this worked for you....