Hi. There are several questions on this subject and I've noticed that none of the staff have answered the question. Since I, too, was struggling with this issue last night, I thought I'd offer some insight. I was so frustrated that I uninstalled the entire forum and started over again (thankfully it had just been created the first time yesterday) because I've used this mod with success in the past and couldn't figure out what on earth was going wrong!
So here it is: If the ad management mod isn't working, then I'm going to have to assume that you have a different theme installed for your board. My solution was to uninstall the additional themes, install the ad management mod, and then to reinstall the theme. If you do this, you will need to edit the theme with the changes that are outlined in one of the stickies above (so that your ads will work with the new theme) but I found that it was worth the extra effort going into it.
If that doesn't work, as apparently sometimes it doesn't, then it appears that it is something to do with the database. This issue is covered, to some extent, on the simple machines support forum.