You navigate between files and folders as you would normally do in the Windows Explorer.
Depending on how you launched the Windows Explorer you will either get a folder tree and a file list, or just a file list. To control which mode you start in, choose either Open or Explore in the Explorer's right-click menu.
To navigate into a folder, simply double-click on it. If you see the folder in the folder tree, you can click on it there. GMail Drive respects your settings in Windows Explorer, so if you have configured one-click activation of folders, this is what you get.
The file list will display all the folders and files in the current directory of your GMail Drive.
Like your regular Explorer view, you can choose from different file list modes. The Large Icons list mode gives the usual icon view, while the Details mode shows various file attributes in a grid fashion. On Windows Vista you can choose the Tile mode to see both icons and various file details in the list.
Note that the Shell Extension will not show the file attribute column in the file list, if you do not have the Show Attribute column checked in the Explorer's Folder options dialog.