Is this something that you could setup before service and have it fire along with an Announcement Cue or Closing Cue? Or is it something that you need to have ready to go so you can fire it at a moments notice (like an Emergency Message that overlays on top of whatever you currently have sent out)?
If it can be setup prior to service, I would just go to the cue and click Edit Stage to switch over to edit that Stage Display. Then, you can either use the existing text box on that page to write in your message (and put it in large letters and a bright color). Or, you can insert a new text box (Insert Object > Text Box) and type in your message and make it look the way you want it.
If it is something that you need at a moments notice, you may need to set up a Key Cue: https://www.mediashout.com/guide/keys
I hope this makes sense! If you have any trouble with this, let us know. This might be a good one to Live Chat about (www.mediashout.com/livechat) when you are there with the computer trying to accomplish this.