You cannot do this with the google maps iframe embed.
You either need to do it with Google Maps Map Maker (http://www.google.com/maps/mm) or you need to use the Google Maps API (https://developers.google.com/maps/).
The downside of the Google Maps Map Maker is you need to manually go back and recreate your map any time you want to make changes.
I would suggest using the Google Maps API.
This is a great little tutorial that shows you a simple Map with multiple markers using the Google Maps API – http://wrightshq.com/playground/placing-multiple-markers-on-a-google-map-using-api-3/