Hi,
Depending upon where your map is located in your file structure will dicatate what you have to do as I came across a bug in the software which caused me hours of frustration and you may have the same issue.
If your containing page is in the root folder you should be able to simply click on the map in the design view of Dreamweaver and you will find that the edit button should display in the properties panel below, simply click that and away you go. Thats the theory, I however found that in some circumstances I didn't get the map edit button appearing and found the only way around it was to edit the page code and place the
<script type="text/javascript" src="google_javascript/wagmp_maps.js">/*wagmp*/</script>
line which is placed at the end of your page and place it directly after the
<div id="wagmp_map_4" style="width: 300px; height: 300px;"></div><div id="wagmp_directions_4">
lines which would then cause the button to appear, there is also an issue when editing the maps on pages which are in subfolders but if memory serves me correctly moving the above code resolves this issue also, when you save the map the code will be replaced at the bottom of the page by the software so to re edit you will need to move the code again.
I did ask for the software to be rewritten to place the code directly under the map info by default but apparently it has to go at the bottom of the page to ensure that there is only one instance of the line in the event of multiple maps on a single page.
Hope this helps