When creating a Mass Notification campaign, you will have the option to select an Advertised Campaign to associate the Mass Notification campaign with.
The only campaigns that will show up in this list are active Post Checkin Offer Campaigns. If the Post Checkin Offer campaign has finished running or is not active, it will not show up on the drop down list.
For more information on Advertised Campaigns and when you might want to use one, please see the Create a New Campaign product documentation and search for Advertised Campaign.