Start out in the Finder level of the computer.
Notice the Go menu at the top of the screen.

Under the Go Menu highlight the iDisk item
To the right will fly out a menu. 
Click My iDisk

If you haven't put in your MobileMe information in the System Preferences this window will come up.
Type in your Member Name (user name@me.com)
Type in your password.
Click Connect button on bottom right.

The window that comes up is your space on MobileMe. You get 20 GB normally and you can add space if you need it. Unless you change the distribution, 10GB are for Mail and 10GB are for iDisk. You can put any files you want into these folders. The Public folder allows you to share files with ANYONE. They can see the files at http://public.me.com/username (username being your user name). 
Like any file on your Mac, all you need to do to share the file, is drag and drop it onto the folder you want. Then, from any other computer you can access this folder and retrieve the file.

QED

If you want to see any of my public folder, go to http://public.me.com/mobeus