I have seen this problem occur before when the page you are trying to add the map to has the head or body tags in uppercase instead of lower case:
Instead of <body>
usually this happens with pages that where not created using Dreamweaver.
try using Dreamweaver to create the page, go to File -> New to create a new file using Dreamweaver. Save this new file to your site before continuing, then try to add a map to it.
I have also seen this happen on occasion when Dreamweaver is in either Split or code View.
Set Dreamweaver to design view, place the cursor in your design where the map should be placed and try adding the map to the page.