You need to be logged in with a valid Google account first - i think.
Then you'll see a red EDIT button top left, click that.
Now a toolbar appears on the map with 3 icons, first a hand icon then a marker icon.
Click the marker icon then click the map to add your marker.
Add a title and optional description - you can customise the marker icon by clicking the icon in the infowindow and selecting a custom icon.
Finally click Save button (top left).
Martin.