The following assumes that you have a new style web site on the MAG server where you log in as yourself, with your own personal user account name and password. There are still a number of accounts that aree accessed by a generic account and password specific to the web site, not the person logging in. Anyone issued with a webmaster account in 2010 or later will have a personal account.
When you first log into your MAG server account, you will see that there is a folder in your home area named for your web site. If you look after more than one web site you will see more than one folder. Each of these folders leads to a separate web site. Under each web site folder there are a number of sub-folders. The one of specific interest to webmasters is the "html" folder. The file index.html in this folder is the top page of the web site.