How do you open the pen drive using command prompt?

You can't work from inside the directories in your pen drive, but you can interact with the contents of the drive, for example "notepad G:\Text\textdoc.txt" will open textdoc.txt with notepad, you can view the contents of the drive with "dir G:"

edit: Actually typing "explorer G:" should open the drive like clicking "View Files" would. Replace G:\ with whatever Windows named your device.