I'm changing your "answers" to "comments" to make this easier to read...see below.
Hi,
ads are not showing up..Please see.
http://www.dentistdirectorycanada.ca/british-columbia-dentidst-directory/
The above ads is used by kento ads rotator, but it has the popup when choose to open a new tab..Not sure when its not showing up..All I have created with ads,zone and placement..
Cheers
Hi, I took a look at your account's configuration and it looks like the minimum height on your zone is set to 250px but your ad is only 245px tall. Since the ad doesn't meet the minimum height requirement for the zone, it isn't displayed. To fix, just make the minimum height for the zone smaller (245px or less) or remove the minimum height restriction entirely.