did you try coalition message under actions? I'm not too handy with the mission editor but you could prolly do one for both sides assuming that works in multiplayer. Should be easier than having one for each client
*edit* sorry I reread your post and your asking how to set it up to display the message every 5 minuites? I would assume it would be continuous action, time more, coalition message. set time more to 300 second for 5 min intervals.