In Windows, I can access the menubar using the Alt key. How can I achieve the same effect in Mac OS X? Specifically I am using Mac OS X v10.6 (Snow Leopard).
The exact same functionality doesn’t exist on Mac OS X. There's no concept of underlined characters in the menus for quick selection.
You can move keyboard focus to the menu bar by pressing Ctrl+F2. Configure the shortcut in System Preferences.
You can press Cmd+? and start typing any menu bar item label to select it from the Help menu entry. This helps tremendously with the menus of rather complex applications.