This command needs root level permissions to execute.
Im assuming you're logged in and trying this command as a normal user... so, at the same place you did this before, try:
sudo mount /dev/floppy /mnt/floppy
(and type in your normal user password when it prompts)
remember, you need to specify both the device and the directory you wish you mount it under (the computer has no other way of knowing where you want your floppy drive to show up as).