I assume you mean from a Mac and not Windows. From .Mac iDisk section...
If you're using Mac OS X version 10.3 or later, choose iDisk from the Go menu, and then choose My iDisk from the submenu. You can also create a copy of your iDisk on your computer so you can make changes to it at any time, even when you're not connected to the Internet. For more information, see the topic "Using iDisk with Mac OS X version 10.3."
If you're using a version of Mac OS X earlier than 10.3, choose iDisk from the Go menu.
If you're using Mac OS 9, choose Chooser from the Apple menu, then click AppleShare. Click the Server IP Address button, type "idisk.mac.com" in the Server Address field, then click Connect. Type your .Mac member name and password, then click Connect. Select your iDisk and click OK.
If you use Mac OS X, you can click iDisk in a Finder window toolbar. (If you don't see iDisk in the toolbar, you can choose Customize Toolbar from the Finder's View menu and add it.) You can also access your iDisk folders directly from the Save or Open dialog in any Mac OS X (but not Classic) application.