desktop: above fold (by topic title), under 1st post, under 8th post, after last post but before response box, to the right of topics. Sizes are 728x90 centered and 160x600 for skyscraper.
mobile: same, but no skyscraper to the right. 320x100 for leaderboard, 300x250 for all the others. Use ad manager to force sizes or customer css to drive it with adsense. You can google to see instructions. You will need to add manually to the skins.