An ad tag is a VAST/VPAID compliant ad tag. You can learn more about VAST/VPAID in the IAB specification.
To init any ads in your ad schedule, you must add one or more ad tags to your account. Typically you will receive such tags as a VAST url from your ad network partner in a form like (sample from the Google DFP network)
give the tag a descriptive name, which will appear in the list to the left once you saved it
enter the full url of the VAST adTag you received from your ad network partner (make sure to not pre-fill the macros with static values)
optionally adjust the ad timeout (this will increase the time the player will wait for an ad to be returned from the ad server before starting the feature clip. Using a higher value might increase the chances of an ad fill if your ad server does waterfalling/redirects, but a too high value will impact user experience and might turn off visitors)
save the tag
Make sure you don't have any ad / script blocker enabled for *.flowplayer.com pages . AdBlockers often block all requests which have "ad" or "advert" in them (especially if it's an adTag from a known advertising network), which will also affect the save request for the ad configuration, hence the ad configuration might not be stored in our systems.