The MobileMe service doesn't allow you to go from the published HTML pages back into an iWeb project file. On your older Mac you need to go into Macintosh HD/Users/(your user)/Library/Application Support/iWeb, and copy the domain file that's in there to an external drive or somewhere else that you can then bring over to the new MacBook. You'll want to copy it into the same location on the MacBook drive and double click the file to launch iWeb with that file.