In your personal settings you can create target audiences. If and how many you can create depends on your plan level.
Once a target group is created, it can't be edited and will take anywhere from 5 minutes to 2 hours to become available. You can see that your target group is available by selecting it in the scheduling tool.
If you delete a target group, the slot will be blocked for 30 days. The reason for this is to prevent the automatic reading of our database.