Are you publishing to MobileMe? I guess you are...
On your Mac go to Finder menu (displaying at the top) and select "Go" then choose iDisk and then My iDisk
or hit Schift+Command(Apple)+i
Then you will see a window with a lot of folders on your screen (your iDisk). Open the folder called "Web" and then open the folder called "Sites". Drop your sitemap.xml file into the "Sites" folder.
Where you will see a file called index.html and a folder named with your website's name.
(Note that this is different from iDisk/Sites/ folder)
The url of your sitemap will then behttp://web.me.com/username/sitemap.xml
if you have a personal domain.
If you're not publishing to MobileMe upload it how you always upload your website.
Note: you don't need to do anything in iWeb!