I guess you just typed in /etc/fstab on the shell, right? This doesn't work. You have to use an editor like vi to open the file and edit it. E.g.,
will open /etc/fstab in the editor vi.
Read how to use vi here: http://www.rru.com/~meo/useful/vi/vi.intro.html
If this is too complicated, you could use WinSCP to copy the file to your Windows machine and edit it there (but make sure to save the file with Unix linebreaks!).